Not sure if any of those count as hidden gems, but here's some games that I've enjoyed that are cheap at the moment:
Eador Genesis 0,49 (90% off)
Are you a fan of Heroes of Might and Magic / King's Bounty? Do you want a similar game with a ton of content and a challenging difficulty? Then take a look at Eador. Yeah, it's not really quite as good as HoMM/King's Bounty, but it's also only 0,49, so you're almost guaranteed to get your money's worth.
AI War: Fleet Command 0,99 (90% off)
Yeah, this game's been around forever, and is apparently 80% off like every other month, but it's also one of the most interesting 4X games out there, since it's focused entirely on an asymmetric player vs ai campaign where you start out with almost nothing and build your galactic empire to face off against a progressively more aggressive ai. In case you somehow don't already own this game, it's definitely worth a look.
Geneforge Saga 5,99 (70% off)
Spiderweb RPGs are pretty well known at this point, but the Geneforge series stands out as one of their most interesting games. It features a unique world with a mix of classic fantasy and sci-fi genetic engineering. It's also extremely open ended, offering a variety of endings and story paths. Sadly the later games are mostly more of the same and don't particularly innovate, but it's still a fun series.
Oh, and you get 5 (huge) games for 6, which is one hell of a price. The other Spiderweb games are also on sale, but Avadon isn't very good, and I'm not a fan of the newer Avernum games on steam.
Drakensang: The River of Time 4,74 (75% off)
It's basically Neverwinter Nights 2. The setting is different and slightly less generic setting, which helps. The RPG rules are also slightly different, which is a welcome change from all the roughly one million DnD 3.5 RPGs out there. But apart from that, the gameplay is very similar, so if you ever wished Bioware still made games line NWN2 this is definitely worth a shot. The first game might also be worth a look, but the beginning of that one is so painfully boring and generic that I just couldn't get past it, so I don't know.
SpaceChem 2,49 (75% off)
One of the best puzzle games ever created. Instead of puzzles that require you to figure out the one and only possible solution to progress, SpaceChem's puzzles just present you with a goal and let you reach it however you want. The freedom this game gives you is seriously impressive. I'd also recommend it to people interested in programming, since it's similar to that in a lot of ways, while still being approachable. And then you can move to
another game by the same developer, which is literary programming.
It is however extremely difficult. Luckily the difficulty curve is still pretty good, with the game slowly ramping up the difficulty, but eventually you'll probably hit a brick wall. If that sounds like a deal breaker this is probably not the best game.
Dustforce DX 4,99 (50% off)
On the one hand, it's not the biggest sale the game's had. On the other hand, it's also one of the best platformers available on steam. It features Incredibly precise controls, airdashing and a focus on speedrunning with in game leader boards. The gameplay also flows
incredibly well, and SS ranking a level feels incredible. If you've enjoyed games like Super Meat Boy, this is definitely worth a shot.
Hollow Knight 9,89 (34% off)
Everyone's practically yelling about this in the sale OT thread so it's hardly a hidden gem, but fuck it, this is literary the best metroidvania game ever made so I'm putting it here. Everyone with even a remote interest in metroidvanias needs to play this game.