Urban Terror is a free multiplayer first person shooter developed by FrozenSand, that (thanks to the ioquake3-code) does not require Quake III Arena anymore. It is available for Windows, Linux and Macintosh. Urban Terror can be described as a Hollywood tactical shooter; somewhat realism based, but the motto is "fun over realism". This results in a very unique, enjoyable and addictive game. Please post all support issues in the forums for the game.
Great and addictive game! I particularly enjoy a high skill curve every once and a while.