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?

Did you guys tried Endless sky? Awesome game if you love space

Dungeon Crawl Stone Soup Love this traditional roguelike!

Mindustry is a really fun automation/tower defense game

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.

CataclysmDDA is an amazing open source roguelike. Has a bunch of tilesets if you don’t like ASCII

Cataclysm Dark Days Ahead has been mentioned so I’ll add OpenMW to the thread. It’s a open source engine rewrite of Morrowind and requires the base game to run Morrowind but it has the potential to be used to create new games and the devs are looking to expand it to allow the creation of ARPGs as well.

Another worthy mention is Endless Sky, it’s an open source game inspired by the classic space trading and combat game Escape Velocity.

Mindustry is an awesome strategy game that combines Factorio with tower defense.

Sonic Robo Blast 2 Kart (ridiculous name I know) is one of the most underrated games ever. It’s a super polished, high quality Kart game with an insane amount of mods and maps. The online isn’t that populated but the singleplayer time trial is TONS of fun. If you like hunting for medals, it can be a pretty deep rabbit hole. There’s a sequel in development but no release date.

GzDoom is the most popular Doom source port and there’s a practically infinite amount of user generated content. If you don’t know where to start I highly recommend Doom: The Golden Souls Remastered and Castlevania: Simon’s Destiny. They’re good enough to be premium games.

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

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

I haven’t seen any incremental games get posted. They aren’t exactly traditional games, but so many of them are open source like Antimatter Dimensions or Synergism. Bitburner is a unique one as an incremental hacking sim, since it relies on actual scripting to grow and automate tasks.

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

Tux Racer. Hands down.

I really enjoy Thrive, it’s basically a modern and more scientifically accurate Spore!

Teeworlds. There are some great community made modes for it like FNG and DDrace. Be sure to download it from ddnet.tw to access to extra features created by the community.