Super tux kart

Vintage story. Much grittier minecraft with a larger emphasis on survival

I’m surprised no one mentioned it yet, but Kitten Space Agency

It’s a free spiritual successor to Kerbal Space Program that tries to be everything that KSP2 wasn’t. It’s still in early development by its progressing fast and it already looks great and allows you to do a lot of cool things, and it’s also built using their own engine instead of Unity and has modding support.

And it’s not on steam.

Destroyed

Dwarf fortress is on steam, humble bundle, itch io and free on their website.

Dwarf fortress is an essential gaming experience.

Beyond All Reason (BAR) A spiritual successor to supreme commander made by a team of fans. It's free and had LAN and online multiplayer. Really fun if you enjoyed the source material.

StarSector

Freelancer. Basically abandonware at this point, but tons of community support, with some great mods and servers.

Xonotic

Factorio you can buy it from there website. It's on steam yes but it's also available on there website with (iirc) to get a steam key or link it to steam if you want to do that.

I've heard good things about star citizen

Aurora 4X if you are into very complex strategy games about building an interstellar empire.

Brogue
Universal Paperclips
Liero

2xko by riot games. Not indie but it is a free fighting game through the riot launcher. Sadly, riot been fumbling with it but I hope they give it a few years of love & development. I genuinely love it.

If you can put up with the (unending) bugs and performance issues, there really isn't something that hits white like Star Citizen for me. A friend bought it for me but I would happily pay the minimum $45 when it finally releases in a stable and reliable playable state.

Voices of the Void

Absolute peak game that doesn't really have any contemporaries.

A game with base management, fully open map, random events ranging from spooky to hilarious, minigames to fix equipment, little robot helpers, big robot helpers that require some assembly, but don't need charging thanks to running on nuclear power, and a quadbike you can backflip.

And the Christmas gun.

Armadiilo Run https://www.armadillorun.com/

The Croc remake is Gog exclusive (and on console) and I love it! It's the first single player PC game that I've 100%'ed!

If you are brave enough there is big list of open source clone games.

Most of them are not on any play store.

https://osgameclones.com/

Vintage Story, a Minecraft-like game that focuses on a tough, realistic survival experience. There are climate zones and seasons. Things like smelting ores or growing food are much more involved than they are in Minecraft, which makes for a strong sense of progression when you've finally managed to make iron or build a windmill.

I recently logged back into my telltale account on a whim and was surprised to find that I owned "Back to the Future: The Game". Even though I'm certain I never bought it. It's not listed on Steam or GOG, but I think you can find it as abandonware somewhere.

Have only played a bit so far, but it looks pretty good. It's a typical point-and-click, not a "X will remember this" story game.

Starsector, perhaps the greatest 2D space game of all time.

Dunno if anyone already said it, but Devotion is always a favorite of mine to share. Beautifully well done story for anyone into horror games.

OSU. There's a open source client don't know anymore which ones good.

Battle for Wesnoth, very good high fantasy strategy game. Lots of player created campaigns, and tons of mods

Edit: looking at their website, it is available through steam, but it is also available in a variety of other places.

I've probably played a few hundred hours of SuperTuxKart. It's a mario kart clone except i like it way better than the mario karts I've played.

Mindustry is fun, it's like a space factory builder thing.

I was going to say the powder toy but it turns out its on steam now

SRB2K, or Sonic Robo Blast 2 Kart, is a mod of SRB2, itself a doom mod, that is an absolute love letter to the classic days of Mario Kart (I'm thinking super mario kart or 64). Full of small QoL changes, and with a forum full of mods for any character you can think of, it's an easy pitch if you've ever wanted to play a kart styled game thats better than the original, with online play

Japanese visual novels that never get translated are a big one. A huge chunk of them live on novelgame, DLSite, and DMM. In fact, there are some full fledged Japanese games that are DLSite exclusive. I've tried a few, and some can be quite good.

The Star Trek games on GOG (like Elite Force I and II, and Armada) are worth your attention.

Full Metal Daemon Muramasa is also typically bought on JAST.

There are also a bunch of gacha games that never released on Steam, though the bigger ones are working to change that.