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.

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

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/

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

I focus on non commercial free games that are also (for the most part) Open Source too.

• Luanti: Website | Flathub | PCGamingWiki | Video | Very close Minecraft like clone (note its old name was Minetest for the longest time). Each downloadable subgame requires extremely low space, in the MBytes range! No account or any online servers are needed (edit: off course its needed to download, but not otherwise). Open Source.
• osu!: Website | Flathub | PCGamingWiki | Video | Rhythm style game where you use the mouse. I think the code is Open Source, but not the assets? I'm not sure.
• Taisei Project: Website | Flathub | PCGamingWiki | Video | Fan project in the Touhou Project series. Old school bullet hell shmup from top down perspective. Open Source.

Note Flathub is a repository for Linux. If you are on Windows, go to their website and look for downloads.

An MMO with no micro transactions: Clan Lord

It’s been around since the 90s, the original developer is now a farmer and gave control of the world to the players rather than selling to a larger company.

We could use an influx of newbies, just don’t be a turd.

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.

Starsector is one of my favorites from the last decade.

https://fractalsoftworks.com/

Vintage Story, Voices of the Void

• 0 A.D.
• Beyond All Reason
• Open BOR (beats of rage)
• M.U.G.E.N

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

Dungeon Crawl Stone Soup, a really good traditional roguelike, often considered one of the four pillars of the genre along with NetHack, ADOM, and TOME. (Those are on Steam, however; Crawl is not.)

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.

Live For Speed

Racing simulator that launched in the early 2000s and is still going. There have been some rough times but overall the feeling of admiration is hard to deny for the 3 (mostly 2 now) devs. A couple years ago they updated it to create and allow mods and this year they released a public test for new graphical updates. I don't even play often anymore but I still check for updates almost daily :)

https://youtu.be/mP1OMIq9Ams

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

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.

There are a substantial number of non-commercial FOSS games that aren't on Steam.

Many---though not all---of them are packaged in Linux distros.

Luanti, basically a FOSS Minecraft clone, is one example; that's in Debian.

A number of older games occur to me that aren't either in Linux distros or on Steam. They aren't necessarily the top FOSS games, but people may not be aware of them, if they rely on looking in distro packages or Steam to find things.

Linley Henzell, the guy who did Linley's Dungeon Crawl, which later became the well-known roguelike Dungeon Crawl: Stone Soup, had a couple of open-source games that he wrote way back. Overgod looks like it's been packaged and is in Debian. Lacewing is in Fedora, but not in Debian.

Video of Lacewing

Video of Overgod

There's a genre of games based on Paradroid. One of the first games I played on Linux was in this genre---NightHawk. It used to be in Red Hat Linux, but it looks like at some point, it fell out of the package repository, as it doesn't appear to be in present Fedora, and it's not in present Debian either today. Not on Steam, either.

I can't find any videos of anyone playing NightHawk on YouTube.

Abuse is a DOS-era action platformer, heavy on jumpscares, that was open-source and had some of the data files (the ones for the shareware version) put into the public domain. Could use the full game's datafiles too, if you got ahold of them. I think that that used to be packaged and in Red Hat Linux at one point, but it's not in Fedora or Debian these days, and it's not on Steam.

Video of Abuse

Surprisingly, while Empire---a very early game in the genre---is in Debian, xconq is not, and doesn't look like it's in Fedora either. Neither are on Steam. You may have played later hex-based strategy wargames in the genre, like Battle for Wesnoth.

Video of xconq

There is a very large collection of non-open-source-but-free text-based interactive fiction out there. These are games that are all or almost all text, where one writes instructions to the game in a somewhat-natural-language format and has the game respond. Zork is a (not-free, commercial, but you may be aware of it) early example of a game in the genre. Sites like IFWiki and IFDB catalog these. Many of the runtimes exist on Linux distros; gargoyle-free is a graphical frontend in Debian that can display games written for many different engines, and there are various terminal-based frontends, but the actual games are not generally packaged in Linux distros or on Steam. One notable exception---Anchorhead, a Lovecraftian game, was updated and had a commercial (and graphical) release on Steam a bit ago (which I cannot seem to get running on Proton under Linux, though given that it's Electron-based, maybe it's possible to run it natively using chromium...I'm not really long on Electron familiarity). I think that the reason that they haven't seen more popularity recently is partly because of the rise of the smartphone and touchscreens and onscreen keyboards. It's a pain to play them without a real keyboard, which many smartphone users don't have available. If I were going to recommend one particular game that I've enjoyed, maybe Babel, for the TADS interactive fiction engine.

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.

Sauerbraten is always worth a mention for me. It's a quake style open source shooter with built-in mapping tools. Definitely a little dated but the game still feels more responsive and fun than a lot of other games today. I think I would really enjoy tinkering with it if I was still a teenager.

I've been playing a lot of Gyakuten Live. It likely can't be officially monetized because it infringes upon "Love Live School Idols" as well as stealing a few assets from the Ace Attorney games. But, the game has quite an impressive amount of production value, with many emotive character faces, a full custom soundtrack, numerous cinematics between cases, and a lot of artwork to help visualize the scenarios. It centers around what seem like relatively innocuous cases, like a missing treasure, or fraudulent tickets, but (aside from the intro case) tends to dive into some complex character emotions as the motive behind each incident.

The other game that made me think of posting this is Hytale, which looks to basically be an indie, non-Microsoft, pro-modding version of Minecraft.

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.

Phantasy Star Online, especially with private servers

https://ephinea.pioneer2.net/

For Linux: https://github.com/zeroz41/pso_wine/tree/master/pso/pc#readme

EDIT: Lutris works better than that Github repo

Panel Attack is a FOSS clone of Panel de Pon with rollback netcode and modding support.

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.

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.

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

YARG - Yet Another Rhythm Game. Like Guitar Hero/Rock Band (way better than Clone Hero) - open source. Guitar, Bass, Drums, Keys, Vocals.

I think it's still being worked on, but some Devs have been hired for the next big rhythm game - if I understood correctly - so the development has slowed some.

You can get quite a lot of songs for free with it, there are some real bangers and it's tons of fun (or go sail the high seas and get what feels like infinite songs, including those from the popular plastic instrument games). Also runs really well on my potato laptop...