Source: Tweakguides
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Back in 2001 a WWII-themed game called Return to Castle Wolfenstein came out, to much hype and applause. I remember enjoying many hours of first the singleplayer and then the multiplayer aspects of the game. Then, just as RTCW multiplayer was dying, the developers suddenly released a free standalone expansion called Wolfenstein: Enemy Territory in 2003. It was a surprising move, and even more surprising was the fact that Wolf: ET went on to become a very popular online game.