Angband

Cataclysm: Dark Days Ahead

Space Station 14

Shattered pixel Dungeon fancade and IDK if it counts but fightcade.

For years I've played bzflag. It's a 3d tank battle game with a variety of play methods (capture the flag, soccer, -em-up, etc).

Other than this, there was once some kind of space opera with combat, politics, and other game functions. I can't remember the name now.

I hate that Tales of Maj'Eyal is my favorite Open Source game. I hate that a new expansion is coming out soon and I will instantly buy it and play it and hate myself. Check it out!

OpenTTD and Widelands are also pretty cool.

I used to play Tremulous. That was a blast. Very clever game.

I played Warzone 2100 for a long time. It isn't as good as Total Annihilation (nothing is), but it had some interesting component-based units.

I should try Veloren some day. It seems very interesting.

Unciv

Neverball is a fantastic ball-in-a-maze (Monkey Ball) OSS game

These suggestions rock, I guess my weekend is set!

Mine would be Thrive, not so much due to its current gameplay, though it is okay, but moreso due to an unwavering commitment to a vision. It has gone on for years and is essentially a scientifically close to accurate version of spore. They have almost finished the single cell stage and are working some on the transition to multicellular now as I understand it. I find it to be an impressive project as it started from the disappointment with spore and has evolved into something that I would genuinely suggest as a learning tool in middle school biology class.

Xonotic is really fun if you enjoyed Quake 3 and Unreal. Has an active community as well. Beyond All Reason is a pretty polished RTS using Spring engine.

Project 1999 and Project Quarm. Emulators of EverQuest, which was released in 1999. Official EverQuest is still going strong 25 years later, but the emulator developers (the are several projects) have an agreement to run their versions of the game.

I'm playing Project Quarm version now and spend way too much time on it.

I've had a lot of fun with Brogue.

openarena?

OpenClonk

How is Linwarrior 3D? Has anyone played it? It claims it's an open source game similar to MechWarrior 2.