There’s a CS-like game called AssaultCube which is available in Ubuntu Lucid (10.04) repository.You can get it by searching assaultcube in Ubuntu Software Center.

formerly ActionCube, is a free first-person-shooter based on the game Cube.Set in a realistic looking environment, as far as that´s possible with this engine, while gameplay stays fast and arcade.
Same to CounterStrike,AssaultCube has network mode and single mode.You can add robots and choose the appearance in single mode.It also has guns,grenade,sniper rifles,etc,exclude first-aid kit.

See the screenshots:




The Mainpage: