Great game, FPS, and its free 
Official website: http://assault.cubers.net/
Assaultcube wiki: http://assault.cubers.net/wiki/Main_Page
Assaultcube forums: http://assault.cubers.net/forum/
Description from wikipedia.org:
AssaultCube is a fast paced first-person shooter based upon the Cube engine. Although its main focus is online play, a single player mode exists which consists of computer-controlled bots.
AssaultCube is available, free of charge to download for Microsoft Windows, Linux and Mac OS X. While the Cube engine is free software, parts of the accompanying game media, such as the graphics, are released under other, non-free licenses.
http://en.wikipedia.org/wiki/Assaultcube
Only for Windows, Mac, and Linux. I have been playing it for over a year I really like it
Current version: v1.0.4
Official website: http://assault.cubers.net/
Assaultcube wiki: http://assault.cubers.net/wiki/Main_Page
Assaultcube forums: http://assault.cubers.net/forum/
Description from wikipedia.org:
AssaultCube is a fast paced first-person shooter based upon the Cube engine. Although its main focus is online play, a single player mode exists which consists of computer-controlled bots.
AssaultCube is available, free of charge to download for Microsoft Windows, Linux and Mac OS X. While the Cube engine is free software, parts of the accompanying game media, such as the graphics, are released under other, non-free licenses.
http://en.wikipedia.org/wiki/Assaultcube
Only for Windows, Mac, and Linux. I have been playing it for over a year I really like it
Current version: v1.0.4