Wolfenstein: Enemy Territory

Wolfenstein: Enemy Territory is a free, stand-alone, downloadable multiplayer game in which players wage war as Axis or Allies in team-based combat. It's a team game; you will win or fall along with your comrades. The only way to complete the objectives that lead to victory is by cooperation, with each player covering their teammates and using their class special abilities in concert with the others.

Minimum system requirements: 

CPU: 600 MHz (x86 compatible)
RAM: 128 MB
HDD: ~300 MB