I had a great time with a couple card battlers last year, Cobalt Core and Nitro Kid.

Cobalt Core has a similar presentation to FTL, with a turn-based format instead. Plenty to do in it, great soundtrack, charming writing.

Nitro Kid is on a more traditional 2D grid with an isometric viewpoint. It appealed greatly to my love of 80's settings, but I'd wait for a sale as it's thin on content.

Sebil Engineering has a really fun mechanic I've never seen before. Its like those Hot Wheels tracks you always wanted as a kid but your parents never got you, but even better. I guess its a traffic control game? Anyone have other examples of these?

The Witcher 3 (/s)

the Skyrim total conversion mod, Enderal: Forgotten Stories, blew me away, and it's free if you own Skyrim. Even has its own installer and game page on Steam.

Dominions (Dominions 6 just came out, but it's an iterative game so try 4 or 5 first)

So it's like civilization and battle simulator had a baby, where armies are managed by the unit and there are simulated battles of thousands.

But all the civs are loosely based on different existing mythologies and there's a crazy complex magic system

Oh, and you get to create a god that you can totally battle with.

If you like civ-like games, it's a really unique and satisfying twist on the genre with an incredible amount of sheer depth.

Shadow of the Forbidden Gods - a strategy game where you play as the ancient cthonic entity waking up because the stars are right. Set in a fantasy world where the forces of good slowly become aware of the coming apocalypse and attempt to forestall your return. You have to get past the janky UI and some dreadful AI art, but the gameplay is unique and satisfying.

I've never heard or seen anybody else mention Suzerain. It's like a choose-your-own-adventure political strategy game, which is pretty unique. You are the new leader of a fictional nation wrestling with corruption. Your decisions will affect the outcome. Game is only $6 on Steam right now and is well worth it.

Also worth mentioning Ostriv, a beautiful city-builder in which you build an 18th Century Ukrainian village, complete with individual little villagers wearing their villager clothes. It's lovely and made by ONE GUY, as best I can tell? Also, last I saw, the entire game was somehow under 1 GB, if I remember correctly. It's absurd.

From The Depths -- very deep large vehicle design and combat game, prepare to happily lose months of your life

Barotrauma -- help crew a submarine under the command of the captain to perform missions in an alien open world. fun multiplayer with good in game VOIP

Universim.

Blasphemous.

Fantastic metroidvania meets soulslike game. The art style, the lore, the atmosphere, and by God - the music!

The combat is not super great, but it's capable enough for a metroidvania.

I haven't played the second one, I hear it's kinda hit and miss.

OMG-Z It's a playstation mini for the PS3/PSP.

Warp on the PS3.

Donut County isn't really a nobody game. But I never see it mentioned.

Ostriv, an 18th century city building game. I believe it is just the single developer based in Ukraine.

Live For Speed, this one isn't quite "hidden" but is overlooked by many as it is not on any storefronts. Sim Racing history and still going strong today with the 3 original devs.

Where The Water Tastes Like Wine - Ever play ToeJam & Earl? This game has the same core game loop. You're on a treasure hunt, unlike ToeJam & Earl where you are trying to find spaceship parts. In Where The Water Tastes Like Wine, you meant to collect stories and then share stories with major NPCs. The stories that you collect and share, change over the course of the game. The soundtrack is also really good.

Splattercat did a video on the game.

This game isn't for everyone. It's a very, niche game.

I'm currently totally hooked on Tiny Rogues.

Void Stranger

Can of Wormholes

Paradigm

Superfighters (original web game) and Superfighters Deluxe (on steam)

Really good 2D platform brawler based around weapon drops.

Treasure Adventure Game, maybe? I believe it's free, though there's a paid remake of it named Treasure Adventure World now.
Cutesy "kid goes on adventure" 2d platformer with sailing and some Metroidvania-y or Zelda-y factors. Cute pixel art and the music is very well done.

I'll toss 2 mobile games on the list. Desert Golf and Golf on Mars. No ads. No stupid paid trinket nonsense. Just a couple bucks for the game and a very chill and casual 2D golf game.

Skator gator is a really fun 3d platformer. It has some time trials that I got into when I never care about things like that. It's cute and controls very well.