Gaming

• Ring of Pain is a great roguelike.
• Paradise Killer - Not actually sure how to describe Paradise Killer, but it's one of my favourite games.
• The Red Strings Club - Interesting, short and sweet
• Stephen's Sausage Roll - If you love puzzle games and can't find any that are hard enough for you, it might be the best game ever made. Or the most infuriating.

And then a few that maybe aren't that obscure by now, but I can't not mention them

• A Short Hike - I think possibly the greatest small indie game of all time? Certainly one of my favourites.
• Baba Is You - less extreme, and I don't know if you can really call it a "hidden" gem given how popular it got on release, but it's great.
• Inscryption - Similar, it's pretty well know, but damn is it a good game.

You've already got a bunch of great suggestions, but I'll throw my two cents in too. I think of these as indies or games that have a niche audience, but some of them probably have bigger teams involved.

• Outer Wilds. It's a quiet, contemplative game about space exploration and seems unremarkable until you really start exploring and learning about the solar system you're in. If you play it, go in blind. IMO it's a perfect game.
• Pentiment. You play as a young artist visiting a Renaissance-era hamlet to work on your masterpiece. The game is gorgeous and thoughtful about the decisions the player has to make when spending time and influencing the narrative. Stayed with me long after I finished.
• Slay the Spire. Tightly-balanced deck builder rougelike. I've put untold hours into this one on maybe 3 different platforms?
• Wildermyth. Hard to describe this one--it's like an interactive fantasy tale with characters that grow and evolve in ways you won't expect, and dynamically told so no two stories are quite the same. Pretty solid combat mechanics, too.
• Undertale. Does Undertale count as relatively unknown? If so, Undertale.
• Case of the Golden Idol. Very good mystery game with a cool, novel mechanic for solving each mystery. Don't let the aesthetic put you off.
• Return of the Obra Dinn. Another incredible mystery game where you are determining what happened to the crew of a ship that sailed into port with no one aboard.
• Papers Please. Simple mechanics that really make you FEEL like you're -Spider-Man- an oppressed civilian in a fascist/totalitarian government.
• Disco Elysium. Another great mystery game, wherein part of the mystery is: what kind of person were you, and who are you now?
• Tunic. It's like old-school Zelda but with more depth and some serious twists.
• Stanley Parable. It's funny and fun.
• Tchia. It's like Breath of the Wild if Hyrule was a real place and the game devs wanted you to love it as much as they do. Fun and charming in a big way.
• Season. Hard to describe and I haven't finished it yet, but the opening stuck with me. It's about what we remember and what we choose to forget.
• Hollow Knight. Does Dark Souls arguably better than Dark Souls. I was put off by the Dark Soulsness and the aesthetic, but once I got a few hours in I was fully hooked. Another perfect game.
• Deep Rock Galactic. Great fun mining valuables and fighting alien bugs with friends.
• 13 Sentinels: Aegis Rim. A time-traveling anime story game with mecha combat.
• The Forgotten City. A time loop game where you are sent back to ancient Roman time.
• Nonary Games / Danganronpa. Both series are anime-inspired mystery games. If you want something that tries to be more cerebral and serious, Nonary is your ticket. If you want tongue-in-cheek violence, Danganronpa is the way to go.
• The Witness. A masterpiece puzzle game built on an incredibly simple core design. One of my favorite all time "a ha" moments came from this game.
• Citizen Sleeper. You are a synthetic being trying to survive on a space station and evade the megacorp that "owns" you. Your body is breaking down and you don't know anyone, so you have to take it one day at a time and do what you can to survive. Clever mechanics and a really well told narrative.

That ended up being a lot more than I intended to share, but if this convinces anyone to give any of these games a shot, I think they're in for a treat.

Dredge is pretty recent but it definitely impressed me with how it conveyed fear/paranoia in an eldritch horror setting. The fishing was also pretty fun!

noita PC - a rougelike witch Sim full of secrets.

Dead Cells everything - Flawless metriodvania rougelite

The Forgotten City PC, PS, XB - A short time loop game that is just fantastic. I think it started as a skyrim mod

Cataclysm: Dark Days Ahead isn't really indie, it is straight up Open Source and a freaking good roguelike. The DDA variant focus heavily on the realism aspect of survival, but there are forks that try to gamefy it more.

There is also Rain World, which I am addicted for 3+ months now. It is also on sale, but before you buy it, know that you are going to die A LOT, but it really isn't a big issue; it's actually funny most times lol. Great lore too.

And one that I've rooted for a long time and I think flopped hard in its release is Below, but I very much still love it.

Also, one that I played for a bit last week and the soundtrack slaps hard is APE OUT.

How did I forget about Noita? This one is a masterpiece. Crafting a wand just to fucking obliterate yourself out of existence .3 seconds later is just chef's kiss. Also hämis.

Stray
Inside
Inscryption

• CrossCode - Phenomenal action RPG. Combat is fast and explosive, dungeons are very obviously Zelda inspired but with way more puzzles. Packed with tons and tons and tons of sidequests, many of which put unique twists on the combat system to keep you on your toes. Make sure to grab the epilogue DLC.

• FOOTSIES - Minimalist one-button fighting game, with rollback.

• OneShot - Fairly reminiscent of Undertale, if you liked that you'll probably enjoy this too. And like Undertale I don't want to say too much, take my word for it and let it surprise you.

• Petal Crash - Absolutely fantastic versus puzzler, and a perfect entry point into the genre. I wrote a very long review of how in love with this game I am, so I'm just going to link that.

• Them's Fightin' Herds - Another great fighting game, been waiting a long time for this port to bring us up to a grand total of two good fighting games on Linux. Has a lot of really cool features like a big story mode with overworld exploration, a cute lobby system with cosmetics to collect and treasure chests to fight for, a dynamic music system that reacts to the fight, and even a semi-cooperative dungeon crawler mode. Has crossplay with consoles as well. Full review.

• Ultimate Chicken Horse - Start on a nearly empty platformer map, each round everyone adds one object somewhere on the map then you all try to finish the level. Whoever finishes gets a point, plus bonus points for whoever finished first or collected coins that have been placed. Then you add another set of objects and repeat. Quickly becomes hilariously chaotic as you try to figure out how to balance screwing everyone else over while still making sure you can win, only to realize that after a few rounds you have all built a horrifying monstrosity. Has full crossplay with console versions.

• Anything by Zachtronics - A bunch of different engineering puzzle games where you have to write code or build a machine to solve problems. Once you've solved the puzzle, you can see a histogram comparing your solution to everyone else's on a few different metrics, encouraging you to go back and try to optimize it further. I recommend Opus Magnum as the best entry point.

The Messenger. Easily my favorite 2D platformer. It starts as a more modern and polished take on the original Ninja Gaiden games for NES, but it becomes so much more.

Inscryption. A rogue-like deckbuilder. You'd think that would make it similar to Slay the Spyre. But again, it becomes so much more.

I recommend both games any chance I have. They are very different, but both are better expecienced blind.

If you like space, you should play Homeworld and its sequels. Incredible game.

If you like chill strategy, try Terra Nil, Islanders, and Terrascape -- they're all really fun and Terrascape has turn-based multiplayer!

If you like puzzle games like Myst, Cyan has Obduction and the slightly controversial Firmament, which I thought was a lot of fun. Quern - Undying Thoughts is in the same vein and feels a lot like Myst or Riven.

If you like Halo's multiplayer with an added Portal gun, try Splitgate. The developer isn't going to be adding more content as they are shifting focus to a new game, but it's still really fun.

If you like boomer shooters, try Amid Evil it feels like Quake, Unreal, and Hexen/Heretic. It'll get your heart racing for sure.

The Long Drive is a cursed game that is surprisingly addicting. Basically it's you, your car, and a 5000km road. That's it. The world is infinitely generated and you don't have to do anything you don't want to do. There's no end game so to speak, just you and the road, if you decide to follow it.

Noita is a very difficult roguelike where "losing is fun" except that it's not always, but many deaths are a learning opportunity, and I find myself starting over even on runs that I live for more than an hour. You have very few healing opportunities, and once your HP gets to 0 it's game over. The game is not fair, but sometimes RNGesus gives you the tools to be a god early on, and even then, it can still be easy to die. Even in full-on god tier runs, there are some things that the player cannot survive. Each playthrough is different, but the map gen is 'basically' the same each game. It's definitely worth your time.

I don't know what's considered a "hidden gem" so I'm just going to shout out all the indie games that I think are at least 8/10 and recommend:

Rimworld

Dead Cells

Dredge

Inscryption

7 Days to Die

Mount and Blade: Warband

Factorio

Mordhau

FTL: Faster than Light

Raft

Gunfire Reborn

Into the Breach

Hollow Knight

Dome Keeper

DUSK

Golden Light

Vampire Survivors

Void Bastards

I really enjoyed Songs of Syx. It's a city builder and economy game, but with a lot of extra detail that you don't often see.

Cassette Beasts! It's a monster collecting game. It adds a lot to the genre (definitely not a clone) and the music is awesome. It got amazing reviews so I'm not sure how hidden it actually is, but I really enjoyed it!

Does Tunic count?

Best to jump in without spoilers, but it's way more than a Zelda/Souls like. So. Many. Layers.

And at a similar depth, but from many years ago. The Ur-Quan masters (space game) https://sc2.sourceforge.net/

night in the woods!

Distance is an arcade racing game with a neon cyberpunk aesthetic. And the main campaign has elements of horror.

It's one of the most unique games I've played - it's almost more of a platformer than a racing game. And the main campaign is worth the asking price alone, but there's 2 other campaigns, a bunch of standalone maps, a level editor with workshop support, a random track generator and multiplayer. Even as someone who really doesn't care for racing games, I absolutely adore this game, and it's criminally underrated.

Diaries of a Spaceport Janitor is one of my favorite indie games of all time. The city design really captures the feeling of wandering around an unfamiliar, large, bustling place. The diary mechanic at the end of the day is a great way to get in character, and I like that you can decorate the apartment. I did some light data-mining (mostly item info and dialogue strings), and I even have fridge magnets of some of the pixel art!

Depanneur Nocturne is also a great evening’s worth of exploration and vibes, but I mention it because I has a reference to Spaceport Janitor and it made me SO happy when I realized that. :)

Road 96 was really fun and the soundtrack was great.

Lost Pig And Place Under Ground

An old-fashioned text adventure, one of the best ones I've played.

Pig lost! Boss say that it Grunk fault. Say Grunk forget about closing gate. Maybe boss right. Grunk not remember forgetting, but maybe Grunk just forget. Boss say Grunk go find pig, bring it back. Him say, if Grunk not bring back pig, not bring back Grunk either. Grunk like working at pig farm, so now Grunk need find pig.

Check it out

My wallet hurts, but my steam library might be able to help.

• The Dungeon Beneath is a turn based party building roguelite. Easy to understand but can get pretty challenging. Also the music slaps.

• 20 Minutes Till Dawn has a spin on the Vampire Survivors formula that I quite enjoy. Merging the crazed progression of items and swathes of enemies, with a twin stick shooter.

• Rodina was what I played when elite and nms were disappointing, and star citizen was still pretending to aim to deliver a final product. For a 1 person team, it really impressed me.

• Get to the Orange Door is a rougelite spin on Titanfall 2 in a weird cyberspace world. The biggest missing feature is a grappling hook.

• Sun Haven is a little buggy but a more rpg and combat focused Stardew Valley. I used to play a lot of rune factory 3, so I enjoyed it.

I tried to recommend some stuff I thought other people wouldn't have played, so I hope you enjoy!

These are all fast, violent and kind of similar to each other:

• Broforce is a platformer with fully destructible levels where you save the world from Satan's terrorist army. It has more explosions than a Michael Bay movie.
• Hotline Miami and its sequel are tactical top down games about shooting up houses. You die a lot but restarts are automatic so it's no big deal.
• Katana Zero is like Hotline Miami if it was a platformer. You're a cyber ninja who can slow down time and reflect bullets with his katana. The story is interesting and very emotional.
• Ghostrunner is Katana Zero but 3D.

Noita. It's just so good. Easily in my top 5 games of all time.

UNSIGHTED: a great Zelda-like/metroidvania from my home country Brazil, with a bunch of cool ideas about how to traverse the world and dungeons, neat souls-like combat and an anxiety-inducing time limit mechanic. It's set in an apocalyptic world where lesbian robots save the world with the power of love and incredible violence.

ZeroRanger: incredible shmup with awesome soundtrack, delicious anime-vibes, insanely-hard-but-fair gameplay and a surprisingly deep lore leans heavily on buddhist philosophy.

Dandara: another great metroidvania from Brazil. This one has an unique movement mechanic - you don't walk, you almost-instantaneously jump between platforms. The setting is pretty unique, with lots of references to actual brazilian culture and history.

One Way Heroics: an addicting top-down roguelike where you can only walk right, lest you be consumed by the Impending Wall of Doom that's coming from behind. Cooler than it seems.

Hacknet: a text-adventure-kinda-thing where you're a hacker doing hacker things. The interface emulates some old 80's systems and all your commands are actualy Linux commands. Also, the soundtrack slaps.

Hypnospace Outlaw: have you ever wanted to be a moderator forum from the late 90's/early 00's internet? Well, now you can.

20XX: Mega Man meets Rogue.

Brotato is a fun, mindless game if you like bullet hell games. Omori is a funny, wacky rpg with some horror elements. Good story to boot as well!

A bunch of my favorites have already been listed, so I'll just mention the one that wasn't: Antichamber, a first person puzzle game that's probably somewhat like Portal in terms of how it requires you to rethink your assumptions about how space works, but it's a very different game, both mechanically and in tone. I don't want to give away too much, but it's a mix of weirdly unsettling elements (although it is by no means a horror game), a design that's actively trolling you in ways that will make you laugh, and mechanics revelations that will have you scream "Wait, I could have done that this whole time?!" It's one of those games that I wish I could delete from my memory and play for the first time again.

You want short and weird? Check out Crypt Worlds. Free too.

You want a bit longer and weird? Check out Hylics 2. One of the most unique aesthetics in gaming.

You want a very streamlined 3D version of Dwarf Fortress? Try Going Medieval. I have over a hundred hours in it.

You want a Czech version of Duke Nukem 3D with the art style of Quake? Well that's very specific. But good news, you should check out Hrot.

These are all indie games that I think aren't quite as popular as they should be. Not the most explicitly unknown, but definitely under-known.

-Super Kiwi 64

Retro 3D platformer in the vein of Banjo Kazooie.

Spelunky HD

A roguelike platformer. While it lacks any meta-progression (like the similar Rogue Legacy games), the core platforming keeps me coming back again and again. The systems-based nature of the game keeps things fresh through many play-throughs.

Umurangi Generation

A photography game that's more cyberpunk than the game titled Cyberpunk. It gradually introduces you into its neon-soaked, Neon Genesis Evangelion-inspired world where humanity fights the good fight against kaiju.

Florence

A story of love, loss, and moving on.

If you like RW/DF, look into Factorio, Satisfactory, Oxygen Not Included. They're all sim/logistics games with different focuses. Factorio is more logistics/big factory minded. Satisfactory has more exploration. Oxygen Not Included is very physics based, you'll need to understand heat transfer by mid game.

I like Deep Rock Galactic a lot more than I thought I would. I don't usually like FPS's. It's coop only. There's mining, and the terrain is 100% destructible. The movement powers are amazing! I love the platform gun. Also, playing dress up with my chunky, ugly dwarf is the BEST.

Hardspace Shipbreaker is amazing. It's shorter than I want, but it's SO well done. I want more of it! You play a disassembler, taking ships apart and chucking the parts into their respective bins. However, the dystopian management has zero worker safety, so it's not so simple. The music is amazing. The voice acting, superb.

As a big metroidvania fan, I'd like to introduce a few of my favorites that go beyond the obvious choices like Hollow Knight and Ori (which of course are excellent). All of these are straightforward and while difficult, they don't focus on confusing menus or min/maxing statistics.

Astalon: Tears of the Earth: If you play one game on this list, play this one. You play as 3 characters, each with their own abilities, and can switch at certain save points. You die a lot and have to start over from the entrance every time, but the game gives you plenty of shortcuts and upgrades that you can quickly go back to where you were. The map is great to explore and there's a lot of content to keep you busy like a boss rush and new game+ modes with different characters.

Momodora: Reverie Under the Moonlight is a short but sweet experience. It's difficult but not unfair enough that you want to stop playing, the world map is well designed, and the pixel art is great.

Rabi-Ribi: This game looks like it was made for a very specific (horny) crowd, but there's a surprising amount of depth to the world and the bullet hell boss girls that inhabit it. It's an interesting mix of bullet hell and platformer that somehow makes up for the nonsensical story.

Here are some games I have not seen mentioned yet:

Valdis Story: Abyssal City
A 2D metroidvania, with the best combat in the genre. It has infinite combos, parrys, skill cancels, a magic system, and 4 characters that play fundamentally different. You get scores for the bosses, which really motivates me to learn the combat well beyond just finding one thing that works well. If you have any interest in sidescrollers, then this game is for you. Controller is required thou.

Zero Sievert
Ever wish that tarkov had a singleplayer mode? Well look no further, Zero Sievert is a top down extraction shooter, that is still in early access. But whats already on offer is really solid and a lot of fun. It has that tense atmosphere when you find something good and want to extract, without wasting massive amounts of your time.

Ollie Ollie World
A 2d skateboarding game with a unique control scheme, that has a very high skill ceiling. Its score chase, with online leaderboards, daily challanges, seasons, and so on. I like its music and artstyle a lot, but if thats not for you, you can check out the older entries in the series, that look not as 'out there'.

Not sure if it counts as a hidden gem, but the last indie game that I played through and really enjoyed was Wandersong, a musical platformer where you interact with everything through song. The same dev also put out Chicory, which is like a playable coloring book, but I haven't had a chance to play that one yet.

eastward: topdown-zelda like. gameplay wise, though it's quite easy. the story is charming, with all the quirkiness from earthbound, although the main plot is actually more akin to the last of us 1, for some reason, or at least it kinda felt like that to me. the visuals are absolutely gorgeous, as is the soundtrack.

phoenotopia awakening: an action rpg-metroidvania mishmash, and a surprisingly large one at that. if you're into challenging and backtrack heavy games, this one does the job very well.

crosscode: this one's been mentioned a bunch on this thread, and there's also been a post here about it a day or two ago, but i'll still mention it because it's just that good. another topdown-zelda like, but keep in mind that i've seen it described as a puzzle game masquerading as an rpg, which may be a more apt description.

Some games I recommend:

Alina of the Arena: A roguelike deckbuilder game. It has turn based tactical battles. There are lots of cards and builds you can try, but there is a fair amount of randomness to it. So you have to go with what you get.

Unreal Life: A really strange, surreal adventure game with a talking traffic light. Great visuals and music.

YumeCore: Just a short little game where you beat up people in a hospital. Why? Why not?

Flood of Light: Great little game with pretty much one core mechanic (direct lights). The Engliah translation isn't the best, but I really liked the atmosphere.

Brotato: Is it a hidden gem? I don't really know, but it is a fun "Vampire Survivor"-like game and a huge time sink. Great shop mechanic and large amount of characters and items.

CrossCode! Incredible game, probably my favorite game of all time.

It's so much more than it looks. The music is awesome, and in some powerful scenes, the story is driven forward with musical motifs that make the scenes hit that much harder.

The story is just... really good. I teared up a few times.

The characters are beautifully written and extremely memorable. By the end of the game, you really feel like you've been living with them.

People say they don't like the puzzles, but the game comes with difficulty sliders and you can turn down things you don't like.

You can get it on consoles, or, on Steam, and even GOG!

There is a DLC to this game. If you enjoy the game, DON'T SKIP THE DLC!!! It is THE ending to the game! Some might scoff at "oh wow they locked the story behind the DLC," but no, trust me, it is well worth it. It adds a TON of content and is an extremely well put together finale to the epic story told in the game.

Ctrl Alt Ego

It's like a System Shock or Prey 2017 that has even less emphasis on "conventional" combat and more general problem solving. In some ways I feel like it's what you get if you grow an "immersive sim" out of a "puzzle" game like Portal instead of out an FPS or RPG.

The Return of the Obra Dinn is one of my favourite games. It's an indie game but quite well known so I'm not sure if it counts as a hidden gem.

I really liked The Case of the Golden Idol too, and its DLC.

You can see I like detective games haha..

If you enjoy city builder games you might wanna try Songs of Syx out. You basically build an empire containing potentially thousands of citizens. There’s no way I could possibly do the game justice in a comment, so you might wanna check out a let’s play or the Steam page

If you liked Hades, play Ember Knights. It’s in early access but I legit put 6 hours in my first session cuz it scratched that Hades itch juuuuuust right