These are/have been my favorite open source games over the years.
Only counting complete open source packages, not source ports requiring proprietary/commercial data to run (Duke Nukem 3D, Doom, Descent, C&C/OpenRA, Total Annihilation etc.)
Spent years playing this throughout the early 00's.
A great take on the early Civilization (1 & 2) formula, huge quantity of countries to choose from but little of the overly complicated religion and culture mechanics from Civ V and VI.
Originally a commercial release published by Eidos in 1999, released as open source in the early 00's and has seen some fantastic updates and additions over the years.
One of my favorite X-COM-likes. It running on the Quake 2 engine sadly means there's no destructible terrain, but I really enjoy its take on the Geoscape portions of the genre, letting you manage lots of bases and installations across the planets as well as having a fairly well tuned difficulty ramp (it still feels very different from Firaxis' XCOM games though, sometimes for the better and occasionally for the worse - cover is harder to come by, and it keeps the classic X-COM mechanic of having fairly narrow vision cones for your soldiers, requiring you to use up movement points to look around you). Absolutely love the soundtrack.
This (and its predecessor Nexuiz) does a great job at taking on the Quake 3 Arena format, some really enjoyable weapon designs and levels, though at times it sticks a bit close to its inspiration.
A cool Elite/Frontier style space sim. Had the ill-fortuned spinoff Jumpdrive/Paragon, but the core version is a really good take on the Frontier formula with a vast universe to explore.
Another arena shooter, not as much a fan of the weapons as in Xonotic, but this one has some incredible mobility instead.
Based on the Spring engine, which originally was a port of Total Annihilation.
