• osu!lazer: One of the best open source (rythm) games imo
• Pioneer: A cozy space sim
• JStris Just a multiplayer Tetris website
• Not "open source" per-se, but games on Tic80 have the code available>

I like simutrans a lot (maybe the most elaborated TTD-like transport game), and I am trying to learn freeciv, but it is a bit "technical". I also like minetest, I have constructed a lot of big houses, the only FOSS game where you can cook a lot of recipes.

It's OpenTTD, hands down. I think source ports and game reimplementations are where open source shines its best.

Start from Doom. Well, not quite the original release, but rather Doom Legacy. From this base, create a 3D Sonic platformer styled after the Genesis era games. That's Sonic Robo Blast 2 (because yes, it's a sequel from a previous fangame named Sonic Robo Blast from all the way back in 1997).

We're not done yet. Mod it further... into an online kart game. That inherits the same modding abilities Doom Legacy has. You've got Sonic Robo Blast 2 Kart, sometimes nicknamed MUGEN Kart to the dismay of its developers due to the sheer amount of addons for increasingly weird characters its forum hosts. If you're interested, said forum, hosting both the addons for SRB2, SRB2 Kart and the SRB2 Kart releases themselves can be found here (for some reason the main website is down as I'm writing this. Oops.)

For SRB2 Kart in particular, the thread to get your hands on it is here and you can find its repository here.

The game is wacky, very fun, runs on a stale potato and it's easy to set up a server to subject an unsuspecting community to your specific choice of insane mods among the hundreds if not thousands available.

My personal favourite is Dungeon Crawl Stone Soup, although it's quite tricky to get into and still brutally difficult, even if you are in it.

It's similar in concept to Shattered Pixel Dungeon, which others have mentioned here. Maybe try that one first. 🙃