Urban Terror™ is a free multiplayer first person shooter, that (thanks to ioquake3) does not require Quake III Arena. It is available for Windows, Linux and Macintosh. The current version is 4.1.
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. No registration required: Download, install, play!
Download from here.