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.
Urban Terror 4.1.1 release for Mac and Linux.
Mac application: ioUrbanTerror.app
Just unzip the zip and make an alias to the ioUrbanTerror application.
Linux 32bits executable: ioUrbanTerror.i386
Linux 64bits executable: ioUrbanTerror.x86_64
You need to make it executable first: right click it, properties and tick the 'allow execution' box.