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.