Ok, not so much as hidden, but also not super known:
Shadow Tactics: Blades of the Shogun. A tactical stealth game (think commandos saga back in the day) is 90%
Technobabylon. A pixelated point & click adventure with some cyberpunk themes at 70%
Return of the Obra Dinn. Probably the best detective/deduction game that I’ve played, at 40%
Prey. One of the best immersive sims, at 75%
The Case of the Golden Idol. Is not as good as Obra Dinn in my opinion, but it’s super good none the less, at 30%
Antichamber. Super good puzzle game, at 75%
The Age of Decadence. Ultra hardcore RPG set in ancient Rome, 60%
Tametsi. Is like minesweeper but good, 67% (and only 0.77€)

A short hike. 99% positive with over 10k votes

A short game too but simply worth it

Definitely In Other Waters, an earlier work from the same people who recently brought you Citizen Sleeper. It’s a narrative heavy game were you play an AI helping a xenobiologist. You perceive the alien maritime world through a quite minimalistic interface, and nevertheless that game had me on the edge of my seat from the start. Highly recommended, and IMO far too little known.

Currently 70% off and less than 5€. Also available as part of a bundle with Citizen Sleeper.

A Hat in Time: 50% off at £12. A really cute 3D platformer.

Wobbledogs: 40% off at £10. Looks and plays like a pet simulator but is almost more about genetics and seeing what kind of bizarre “dogs” you can create.

Let’s Build a Zoo: 45% £9. I’ve seen this described as similar to a Bullfrog game (think Theme Hospital) and that’s good way to put it. It’s a zoo simulator, but full of silly humour and things you can do, like butcher your animals for meat. A big feature of the game is breeding, including crossbreeding animals to create hybrids like a snake/pig monster.

Sherlock Holmes vs Jack the Ripper: 80% at £2. It’s an old game and the dating shows, and nor is it a brilliant masterpiece of writing (it’s Sherlock investigating Jack the Ripper after all) but it does scratch the detective game itch. It’s fun to explore London as it was in Sherlock’s day, and it does provide its own theory as to who Jack the Ripper was, as well as laying out the events in a way that are much easier to follow than reading a book. I’d recommend it for true crime and mystery fans and I look forward to the other games in the series.

My picks for some (actually) hidden gems :)

Risk of Rain 2 - 50% 12.49€ 96% positive
Action rogue lite hero 3rd person shooter thingy - My absolute favourite game in recent memory.
Not sure if this counts as hidden but many fans of the genre don’t know it ¯\_(ツ)_/¯
Insanely good soundtrack, cool artstyle, creative characters with very diverse abilities and gameplay, plenty of enemies and bosses, hidden features, locations, items, bosses, c̴͕͎͚̗̥̀̃̓h̴̘̯̎͗ȧ̶͕̤̪̦̺̂̕r̷͚͑̔͒̃͝a̵̖̩̣̍̂̈́̎̿c̷̱͓̪͖̈́ṫ̵̯̈͂̚e̷̺͓̙̥͆̓ŗ̷̢̻̺̞͠s̸̩̍̍̀?̴̼̘̱̗̯̈̋̄
You know how some rogue lites make sure items stop stacking so they don’t break the run?
Yeah that’s not one of these. Go nuts, break the game, fly through the map, just to get too cocky and lose it all in the blink of an eye.
The original Risk of Rain is also a banger, currently 80% off at 1.99€. Same concept but 2D sidescroller with pixel graphics. Incredibly sweet game.

Art of Rally - 45% 13.74€ 92% positive
Challenging game with arcadey look and sim-ish feel.
Beautiful visuals and a chill soundtrack. Great for white knuckled time attack or hours of zen drifting in freeroam.

Devil Daggers - 50% 3.99€ 96% positive
Highscore based retro arena shooter bullet hell kinda deal. PSX era 3D look, one weapon with two modes, surprisingly interesting movement (think b-hops, rocket jumps).
Every run is mostly identical. Enemies always spawn at specific times, but in random locations of the claustrophobic empty map. The longer you survive, the higher the score.
Also fantastic sound design!

Hyper Demon - 33% 8.36€ 95% positive
Sequel to Devil Daggers. Turns everything to 11. Complete fever dream. Adds an absolutely psychotic rear view mechanic, where close enemies outside your FOV appear mirrored in red colour on your screen.
Adds several movement and combat mechanics, while still keeping the weapon very simple.
Different score system while keeping the timed spawns. Killing in rapid succession increases your score, just scraping by drops it over time.

Lethal League Blaze - 35% 12.99€ 97% positive
Projectile Fighting Game with saturday morning cartoon style 3D visuals. There’s a ball in the middle, whoever touched it last “owns” it. If you get hit by the ball, you take damage. If you hit it back, you own it and it speeds up. And you build special meter for absurd special moves.
Turns into a mind melting firework of on screen effects the faster the ball gets.
Has a decent campaign to learn the game and fight bots. Bangin soundtrack
Up to 4 players, local and online play.
The original, Lethal League is also on sale for like 5.99€. Fully 2D visuals, also very charming. iirc slightly lower health and no campaign.

Nidhogg - 60% 3.99€ 90% positive
If you don’t know Nidhogg go play Nidhogg. Bring a friend.
Low-Fi 2D fighting game.
You can run, you can jump, you can duck, you can stab. 4 Maps to utterly cover in pixel blodd.
That’s about it.
It opens up more the better you get, but just go play it, folks.
Win against your buddies by getting through 3 screens in a tug-o-war kind of system and prove your worth by sacrificing yourself to the Nidhogg.
Tournament mode available.
Nidhogg 2 is also on sale for 5.99€ 81% positive at like 800 reviews.
They got budget for graphics that time which… they used.
Different weapons add a rock paper scissors dynamic to the game, everything else is mostly the same! Good stuff.

Post Void - 20% 2.39€ 97% positive
A wild ride of a psychedelci action rogue like? No meta progression from what I could tell, which is why I’d be hesitant to call it a rogue lite.
Basically run based randomly generated doom while your ego melts away after a heroic dose of psychoactives.
Health literally runs out (of the head you carry) over time. Kill to heal, don’t get hit.
DO NOT TOUCH IF YOU HAVE EXPERIENCED PHOTOSENSITIVE EPILEPSY BEFORE.

Redout - 75% 4.99€ 87% positive
The quintessential zero-G racer for PC.
Very similar to the classics like Wipeout, but innovating on the formula. You’ll need a controller!
Left stick steers as you’d expect, right stick points the nose of your racer to compensate for tight turns and to prevent blackout / redout during high-G maneuvers.
A classic campaign mode lets you unlock several classes of racers, two handsfull of upgrades let you adjust how you play.
Sweet thumpin soundtrack Online play supports up to 12 players though I’d be a bit surprised if you find that many on the servers :D
Split Screen mode available!
The Sequel Redout 2 is also on sale at 14.99€ but it has even fewer reviews, fans of the games seem to favour it for longer tracks, deeper customization and improved gameplay. I haven’t tried it yet though.

Heat Signature - 50% 6.49€ 94% positive
If Hotline Miami was set in space. And a rogue lite.
Pull off crazy heists aboard enemy ships to liberate the galaxy from authoritarian empires, each faction generally having some specializations when it comes to gadgets and tech which you get to unlock for your runs.
Can be frustrating at times but on the flipside a close save get so much more intense. Also you usually get several chances to save your skin - if you’re captured you get thrown out an airlock and take control of your escape pod to fish your body out of hard vacuum.
Generally there are so many cool mechanics, I’m having a hard time to pick highlights.

A few to add in:

Solasta: Crown of the Magister - Old school D&D and tactics RPG. It’s indie so has rough edges here and there, but it’s so fun, and there aren’t a ton of games that fill the void.

Dungeon of the Endless - It’s a tower defense / tactical game with great pixel art and one of the best soundtracks I’ve ever heard.

Battle Chasers: Nightwar - Excellent RPG that will remind you of old school Final Fantasy.

Tower of Time - Excellent tactical RPG

Detroit: Become Human - Cinematic narrative game about androids becoming self aware.

The Dark Anthology: Man of Medan - Cinematic narrative horror game, the first in a series of several, therefore if you enjoy there’s a lot more where that came from. (Made by the same studio that created Until Dawn.)

EXAPUNK - 50% - 96% Positive

If you like old time puzzle games, and have a pinch for programming, then you will love this. In this game you control bots by creating algorithms to extract data and other challenges. The cool part is you must study the game language and learn the lore from manuals and magazines that the game provides.

SHENZHEN I/O - 50% - 93% Positive

From the same creator of EXAPUNK, only the thematic here is electronics.

Road 96 - 50% 91% Positive

Summer 1996, Today is the day! You hit the road. Adventure. Freedom. Escape. Run. Flee the Regime. Try to survive.

On this risky road trip to the border, you’ll meet incredible characters, and discover their intertwined stories and secrets in an ever-evolving adventure. But every mile opens up a choice to make. Your decisions will change your adventure, change the people you meet, maybe even change the world.