Gaming

Oof, I have a huge boner for roguelikes, and most of them are open source. My biggest time sink is still Dungeon Crawl Stone Soup, playable online at https://crawl.develz.org/

Dungeon Crawl Stone Soup

0 A.D. is the closest experience to Empire Earth I've had on the FOSS side. Great fun!

The only one Ive played for any length of time is freeciv, which as the name implies is based of the civilization games, mostly civ2. It's a bit old looking these days and setting up multiplayer has always been a bit fiddly, but the gameplay is pretty fun imo.

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.

osu!lazer

Simutrans. It's old but really good. It's really easy to lose 10h or so in it before you notice the time.

Pokemon Emerald 😉

Battle for Wesnoth is a personal favorite of mine that I haven't seen mentioned elsewhere in the thread. It's a pixel art, hex-grid turn based strategy game with fantasy armies. It's mixed with mild RPG elements, and there's tons and tons of campaigns and fanmade content accessible directly through a mod browser in game.
The other games I enjoy that I see being mentioned here are in the same vein. BAR, Warzone 2100, OpenTTD. And the open mods based on the STALKER engine, like Anomaly. I love those.

I am a big fan of Neverball

I like supertuxkart and minetest

Veloren

Doom :)

Dungeon Crawl Stone Soup for me. A classic roguelike with active community and regular updates as well as several forks. It also has a good amount of playable races. From classics like elfs or orcs ~~and dwarves~~ (RIP Mountain Dwarves you were too good for this game) to funky races like Vine Stalkers (sentient parasitic plants) and Armataurs (centaurs but with armadillo parts instead the horse ones) to cats and octopuses.

And the gods you can worship in the game are also very diverse. How about a slime god? Or someone who wants you to wear as much cursed equipment as possible in exchange for knowledge? Or maybe the one who wants everything to move veeery slooow and rewards you for killing mobs that are faster than you?

OpenTTD is a great open source clone of the transport tycoon game. And boohu is an awesome ASCII coffeebreak roguelike.

I'm surprised to not see Flare RPG mentioned. It's nothing groundbreaking on its own, but it's actually got two nicely fleshed out campaigns, and tooling for people to make new ones. It's a nice, fun, FOSS single-player RPG, and it's great if you want that old fashioned Diablo feel.

Another vote for Endless Sky here as well, it's just excellent, and surprisingly expansive.

Only one that comes to mind is Lugaru. The creator had this weird legal battle where someone stole the game, ported it to iOS, and the thief claimed because it was open source he was allowed to sell it.

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

InfraArcana was a little bit tough but is a very good roguelike

Pokemon Auto Chess. Literally what the name implies, a Pokemon auto battler game. Honestly it's really well-polished for something only a couple of people are working on (aside from commissioned sprite artists) and has an active community to go with it.

Thanks for the Veloren recommendation, I liked Cube World, it just didn't work out well.

Warzone 2100. It reminds me of the older Command & Conquer games with a ton of upgrades and customization for your units.

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.

OpenTTD is my drug. I'm addicted

shattered pixel dungeon

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.