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.
I really like this game, I've played it a lot and I'm still playing it now and then. For a fast first person online shooter this is the best and it doesn't require you to have a great computer either.
I give it 9/10 because of all the gaming joy I have received from this game.