Since it is so hard to find a truly Open Source FPS, here's a list which should clear things up:
- FreeDoom - everything modified BSD.
- Nexuiz - everything GPL (However, the tradmark and engine were relicensed for commercial use)
- Xonotic - Everything GPL (Based on Nexuiz source code after reliscencing)
- Open Quartz - everything GPL.
- Red Eclipse - Based off Cube 2 engine, under zlib, content under Creative Commons Attribution-ShareAlike.
- Terminal Overload - code under MIT license, content under Creative Commons Attribution
- Tremulous - engine GPL, content mostly Creative Commons Attribution-ShareAlike.
- TUER - code source under GPL, artworks without licence
- Cube - Partly open source engine (network code is not, neither is the content)
These games use an Open Source game engine and/or gamecode, but the content is only free as in beer, not freedom in the true sense of the word; they are effectively more like freeware games.
- Sauerbraten aka Cube2
- World of Padman
- Wolfenstein: Enemy Territory
- Smokin' Guns
We'll just list them, as they are not important for our project:
- Americas Army
- F.E.A.R Combat
- War Rock