Urban Terror is one great game that deserves more attention. As someone who recently tried it, I'm impressed by what the FrozenSand team has accomplished with this free, open-source first-person shooter.
What is Urban Terror?
Urban Terror is a free tactical shooter built on the Quake III Arena engine, released under the GPL license. It's a standalone game that doesn't require Quake III to play, and it runs on Windows, Mac, and Linux. Think of it as a middle ground between Counter-Strike's tactical gameplay and Quake's fast-paced action.