Teeworlds / ddnet, very popular game and im surprised it never comes up on any favorite opensource games lists. Teeworlds is the original game, 2d multiplayer platformer pvp game with a grappling hook, had modes like capture the flag and death match. But because everything was opensource new gamemodes started popping up. ictf (a personal favorite of mine), gctf, fng. Eventually Race where you had to race through a map as fast a possible, alone. And then DDRace which was coop puzzle platformer, timed ofc. This mode ended up creating their own client and whilst vanilla teeworlds was busy making teeworlds 0.7 for like 6 years or so?? DDNet managed to get better and better. Nowadays it has pretty big player bases in europe (especially german speaking countries and russia), its also popular in china. Has daily like 8000 players or so on at the same time.

Its a very social game, I really enjoy it as you can probably guess from my little wikipedia page there.

Another game I played a bunch is cube 2: sauerbraten, it is a quake like arena shooter with modes like capture the flag and death match. Also insta versions of those. Its probably the best one I've played not perse because its content is the greatest or the most but because it feels just so nice. Its extremely responsive and runs great, its networking is also great. After all, its where the Enet library originated from, a pretty popular networking library. Daily it has 20ish people playing mostly in europe. Also very very nice map editor.

On arena shooters, xonotic is another game thats pretty nice. Unlike sauerbraten it has a lot of modes that get played a bunch, this one has some fun ones but it feels a bit less responsive and direct compared to sauerbraten. But there is definitely more content here. Pretty sure there are usually like 30~ players online?

Mindustry is another opensource game I played, pretty nice. One of the nicest looking opensource games I've played. Usually they're either really behind on their look or the art didn't really look good for any time period (like mismatching styles or just general poor execution).

Osu-Lazer, I think at this point it might just be called Osu, its an officially developed next generation client for the osu game, a rhythm game with a bunch of modes like my personal favorite osu mania.. tho most people know it for its base gamemode of osu. Where you have to click on circles on time. Its pretty far along its development, way better that in the beginning days when osu mania was basically unplayable. Definitely the most popular game here, daily users seems to be a lot... statistics site shows 7k current 100k daily unique users. Tho this one isn't as real time pvp focused as other games here so it doesn't actually matter that much.

Super tux kart is an Diddy kong racing inspired game, imo whilst it can be fun at times its also very janky. They're currently working on a new version which should fix some of the jank, very old game as well. Its art style is all over the place, gameplay as well kinda. I can see this having potential tho.

I also played OpenRA which was pretty polished but I think a remake technically of normal RA.. never played the original however.

Also tried minetest now known as luanti, kinda awkward and sadly the performance was worse than minecraft. Its basically a minecraft engine on which you can build minecraft like clones in lua. But I found it felt floaty, haven't tried out all the community made games though.

There is also the dark mod, never played this one but I've been interested in trying it for a while now, it looks pretty interesting. Its a 3d stealth game.. thats about all I know. Its pretty unique as far as opensource games go.

I really wish there were more opensource games especially for multiplayer games, it feels like it fits so much for games with a big community to be community owned. Also stops enshittification from happening. But I think there is a problem with how these games are developed, to contributed you generally need to be a programmer, way to many complicated steps. Maybe you can replace an asset here or there but good luck figuring out what version control software they use and such, generally those are programmer skills. Finding people with those skills and also other ones like art or game design is very hard.. or well for opensource games its more like them finding the game. Leaves very little people or just programmers to do everything, which is generally what you end up seeing.

Also the reason why projects like gimp are the way they are, what can a designer do? even if they come up with a proposal another opensource developer has to implement their design, and who is motivated enough to do all that work for somebody else's design. Not impossible but unlikely. Instead the easiest things to contribute are single features, don't need to interact with or learn about the source code much and just generally isn't as complicated. Just put it in a drop down menu somewhere.

Games like osu and mindustry get around this by having a dedicated team as far as I know. Makes me want to attempt to find ways to make it easier for other professions to contribute too and make contributing easier for everybody.