Urban Terror is a realism based total conversion mod for Quake III Arena which no longer requires this game to be played. It only uses its engine, which is open sourced through the GPL and may be distributed freely. Urban Terror plays in about the same setting as Counter Strike, but is less focused on realism and plays much faster, which among other things, is caused by the ability to strafe jump, which combined with wall jumping can lead to very quick movement and nice jumps. Urban Terror has 7 game modes, 25 maps and 15 weapons.
If you have Quake 3 Arena already installed, you only need to run the Urban Terror Installer.