Cube 2: Sauerbraten is a free multiplayer & singleplayer first person shooter with Quake II style game play, supporting many multiplayer gameplay modes like Deathmatch, Instagib, FFA, Efficiency, Tactics, Capture (somewhat like Domination in UT), Regen Capture (like capture, but you regenerate health armor and ammo next to bases), CTF (capture the flag), Hold (hold the flag for a time to score points), Collect (frag the enemy team to drop skulls), Coop edit.
It is supported on Windows, Mac OSX, and GNU/Linux. FreeBSD is also known to work, although it is not officially supported. The engine is licensed under the OSI approved ZLIB license, and the content is covered by a variety of typically more restrictive licenses. It can be downloaded for free from the Official website.