I suck at fps games, but I found my niche in this one. It may not be the prettiest Quake-based game out there, but the gameplay is simple and addictive. I suggest that you try it.
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.