Urban Terror is a free to play First Person Shooter, available on Windows, Linux and Macintosh.
According to official site
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. 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!
In other words, Urban Terror is a fast, deadly, and open-source FPS that focuses to a certain extend on realism, but never at the price of fun.