I've mentioned these before and I'll do it again:

• Exanima - Read about the features. This one is more impressive than the screenshots make it look (at least for me).
• Lunacid - I love the visual style and atmosphere of this. I also enjoyed Lost in Vivo by the same developer.
• Praey for the Gods - This one is for anyone who's looking for more games like Shadow of the Collossus.
• The Upturned - A cartoony horror-comedy game with a great sense of humor.
• Withering Rooms - The story is interesting and the atmosphere is great.
• Your Spider - This one is possibly my favorite indie horror game.

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.

Shadow Tactics and Shadow Gambit are two brilliant gems that come to mind by Mimimi Studios. I discovered them a few weeks ago and just learned they went defunct back in August because they were too niche a genre and couldn't make enough sales. They're Stealth Strategy games where you control a group of ninjas/pirates through a heavily guarded level to the objective, stealthily murdering everyone along the way. If you get seen you can easily jump back to a quick save and try again. You're not overpowered and can easily be killed by enemies so save scumming is deliberately built in to the experience to experiment with your approach.

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.

rogue Lords: it is inspired by Slay the Spire (StS). Card game with roguelike element. Here, the cards are replaced by your minions skill. But the right set of skill is less frustating to build than in StS. Making it a more fun experience, and the graphics are way above StS(not hard). As in StS , with luck/skill you can manage to build some 'infinite deck" where you never let go of the control of the battle.

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.

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.

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.

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

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?

After Hades, I hope some folks went back and played Supergiant’s other titles. I love them all. But even amongst them, Pyre is the underdog, unknown, shunned. And I think it’s fantastic. The music and writing is top notch. You can really see the bones of Hades in all their games, but they polished their world building and story telling to perfection in this one.

I'm currently totally hooked on Tiny Rogues.

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.

The Witcher 3 (/s)

Universim.

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.

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.

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.

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

Really good 2D platform brawler based around weapon drops.

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.

Paradigm