If you havent played this already, you are in for a treat:

https://veloren.net/

https://www.youtube.com/watch?v=eiIuypUoAhc

Mindustry.

Every time this question comes up.

And I'll never get tired of saying it.

• Battle for Wesnoth is one of my favorite turn-based strategy games.
• Dungeon Crawl Stone Soup makes a whole day disappear when I play it.
• Tales of Maj'Eyal is another, slightly more graphically rich roguelike. I haven't played it too much, but it seems like there's quite a lot of content to it.
• The Ur-Quan Masters... I don't even know how to categorize it. It's tough as nails, but the art style is rich, and the worldbuilding draws you in.

Any others I would have mentioned are already in other comments.

• Beyond All Reason (Supreme Commander, Total Annihilation clone)
• Warzone 2100
• Liero
• Hedgewars (Worms clone)
• OpenTTD (especially JGRPP fork)
• GZDoom / Woof / DSDA and other DOOM 1 and 2 source ports

And some that require original assets:

• OpenMW (The Elder Scroll: Morrowind engine replacement)
• OpenXcom (especially OXCE and Brutal-OXCE forks, they all have great mods, like XCom Files, Harmony, XPiratez)

Also, check this open source or source available games list: https://osgameclones.com/

Xonotic is fun, kind of brings back the nostalgia gameplay of DukeNukem and Quake

OpenRA

My personal top 4 favorites, in no particular order, are:

1. SuperTuxKart
2. SuperTuxAdvance ( has a windows download and I'm not smart enough to figure out how to set up and run a version for Linux outside of using WINE or Proton and is pretty WIP )
3. Luanti ( specifically VoxeLibre )
4. Mindustry ( liked it enough to buy it on Steam to.support the devs )

I know OP already mentioned #3, but you know.

Also, if we could include things like OpenGOAL ( which relies on having files from the Jak and Daxter games ), I'd include that since I just got roughly back to where I left off on Jak II before I lost my memory card. Though that's cheating since it relies on copyrighted proprietary files.

space station 14 and Mindustry are both excellent games.

OpenRCT2 is really great, if you count that

CDDA and Veloren are some that I have played lately.

OpenTTD

My favorite open source game is by far Ring Racers

Bitburner

It's a programming idle/incremental game. Has a bit of a learning curve (not bad if you already know how to code) but it is my favorite programming game. It scratches so many itches and it really invites players to explore. There are even secret achievements you can only get by messing with the source code and rebuilding it!

https://pathos.azurewebsites.net/

Pathos is a really good one as well.

Try stratagus and devilutionx and gltron/armagetron and unciv

Nethack by an astronomic margin.

Assault cube + reloaded, ppl are rarely on but it feels good

osu

Xonotic is Nice ig

Pekka Kana 2

Iter Vehemens ad Necem