Enemy Territory is a completely stand alone and free product. It was originally going to be the expansion pack for Return to Castle Wolfenstein... The single player aspects of the game were lagging and lacking, so the game was canceled. But the excellent multiplayer, developed by Splash Damage, was released completely free.
Its a WW2 game with classes, and RPG elements as you level up as you kill/achieve objectives etc. Campaign mode lets you play across all the maps and retain your experience build up...
You can find it here:
http://games.activision.com/games/wolfenstein/