http://sourceforge.net/projects/nexuiz
a quake-based FPS game
Pros:
- it's completely free software, sources to all the maps are available and all the game contents and engine code are under GPL
- Game media files are in Quake3 format, the mod code is QuakeC
- With bots
- Can be run under GNU/Linux, MacOS and Windows
Cons:
- Runs too slow on my computer
- QuakeC is still too limited