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.
A decent F2P FPS.
PROS+
+Decent player-base.
+Fun gameplay.
+Available on Linux, Mac, and Windows.
CONS-
-Not a lot of characters to choose from.
-Not much weapons.