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.
An installer of Urban Terror 4.1.1 for Windows. If you use Vista, you might want to install to another location than 'Program Files (x86)'.