Gaming

There are a few open source game projects I follow. I suppose the most famous one is the Freespace 2 source code project. Although it didn't start open source, the original devs open-sourced it later. It has great support, and a great modding launcher called Knossos. To play the game, even with the source code, you either need the original disks or a copy of the installer from GoG, but it's really cheap. Getting it working on Windows is pretty easy, but Linux is only slightly more complicated. (Fortunately, there's a new launcher that makes it way easier).

If you're interested, let me know.

Cataclysm: Dark Days Ahead

0 A.D. and Beyond All Reasons for me ^^

Technically not the whole game, but (Smash Remix)[https://github.com/JSsixtyfour/smashremix] is an open source rom hack of the original Super Smash Bros. for the Nintendo 64. It's been a blast from the past and I'm enjoying the hell out of it.
I also love the fact that a lot of N64 emulators are open source too, like Mupen64 or simple64

Does City of Heroes count? It was closed source but was opened...

For me it's CDDA, a quite realistic and deep apocalyptic survival game with very frequent updates(on the experimental branch)

shattered pixel dungeon

Openttd

I made one! It's a solitaire-like but with some new mechanics, and fairly polished and balanced at this point. You can find it on itch and on the play store. Here's the source.

Aside from the other choices already mentioned (especially Doom stuff, FreeDoom ftw) any of the free games derived from the S.T.A.L.K.E.R. games and early builds, mostly Anomaly and Lost Alpha. There are also a few stabilized versions of the Build 1935 alpha out there which are worth playing and probably a bit more approachable for most people.

Not really, but kid has been happy with gcompris

Surprised it's not mentioned here, but Bzflag.

Super fun tank shooter game that doesn't take much to run, and reminds me of a cross between the very old bolo game and Mario kart's battle mode.

I've had a ton of fun in Red Eclipse over the years.

classicube is pretty cool, open source Minecraft classic-like game (so, no crafting or survival aspects, limited world sizes)

[https://spacestation14.io/](Space Station 14)

Tremulous - The game was so fun back then (like 10 years ago ?) I guess it's a dead game now.

I used to play openarena (Q3a ish) a lot. And we had some group games of freeciv. I never got torcs to work well enough to play. That would have been great and simulation would be the perfect kind of "game" to free / open source. And countless hours of nethack on every platform I had (which was a few). Other roguelikes too. Ones that actually are like rogue, that is. Even rogue.

But all that was a long ago. Should maybe explore more.