BZFlag is an OpenSource OpenGL 3D networked multiplayer "battlezone capture the flag" tank game for Windows, Linux, Mac OSX, Irix, FreeBSD, Solaris, and other platforms. It's playable with software rendering with better game play on hardware 3D systems. Integrated game server locator.

License: free

Additional System Requirements: BZFlag is an OpenGL application, and as such requires an implementation of OpenGL that works on your system. Mesa is known to work.