AssautCube - An awesome FPS game for Linux
Based upon the Cube engine, AssaultCube is a fast paced, first-person shooter, (partially) open source, cross platform game. Formerly called ActionCube, this game can be played both online with hundreds of other players from around the world or in a single player mode against computer controlled bots.
There are two different teams in AssaultCube, called the Cubers Liberations Army (CLA) and the Rabid Viper Special Forces (RVSF), with 12 different game levels excluding the single player mode. The game delivers a realistic environment while being very gracious on system resources.
Testing it on a 512MB RAM and 2GHz Intel Celeron processor Ubuntu powered box, it run great both in the single player mode and online.