Enemy Territory
It is a FPS free multiplayer game based in WWII.
This link will tell you all about it but from personal experience it is a KILLER game and has very good graphics. There is also LOTS of different maps, weapons, classes (have on specials, for instance the medic can give health and revive people, where as the Field opps(my fav) can give ammo and call air support, the covert opps can change into an enemy's uniform and trick the other team, the engineer can plant mines and bombs, and the solider has a lot of different weapons too choose from such as the flame thrower, morter, or different guns) , and gametypes you can play.
Info http//en.wikipedia.org/wiki/Wolfenstein_Enemy_Territory
Free Download http//www.splashdamage.com/node/57
(The original just Wolfenstein Enemy Territory is the best)