Timespinner: A really solid Metroidvania - very similar to older Castlevania games. It's on sale for just under $6 USD. Great deal. I got a solid 28 hours out of the game and loved it (tho, if you race to the end it isn't super long but still good for the price). Don't sleep on it for that price if you like the genre, and are itching for that old school Castlevania feel. An actual hidden gem IMO.
Broforce: Not a hidden gem, but a hilariously fun side scrolling shooter like Contra that you could play with a few friends (and levels going up and down, not just running to the right, plus destructible environments that you can use to kill your friends for laughs). $3 USD, hard to pass up even if you only play it for one drunken night with your friends. We all had a lot of fun with this. Maybe too much fun.
Toukiden 2: This game has been sitting at full price for years. I could never understand why. It's a Monster Hunter style game with a full open world for single player, but multiplayer gameplay for boss battles similar to MH. I had fun with it for a bit but I have been waiting for it to go on sale so I can play it with some friends. Significant sale at $23.99 USD compared to the usual $60 USD and it doesn't happen often. It's worth looking at. I enjoyed the few hours I played a lot, but I couldn't tell you how good it is overall. Reviews seem good.
Grim Dawn: Others have already mentioned this one. It's the successor to Titans Quest, and perhaps the spiritual successor to Diablo 2. Supports mods, which is great! Stash enhancements, character editors, etc. It's an ARPG game similar to Path of Exile and Diablo. Story is decent, there's some cool mechanics and choices matter type stuff, but even with those you can still play with your friends. You can get the game with expansions (totally worth getting) for around $21-$25 USD. Great deal and you'll be supporting a great company, Crate Entertainment takes care of their fans and keep supporting the game.