Return to Castle Wolfenstein
"Return to Castle Wolfenstein" received favorable reviews from critics. At Metacritic, it scores 88/100 (based on 32 reviews), and on GameRankings it scores 86.75% (based on 50 reviews). "Eurogamer"'s Tom Bramwell called "Return to Castle Wolfenstein" "a worthy addition to the stable of id Software affiliated shoot 'em ups. The single player game is average to good and takes quite a while to finish, but the game really earns its salt by shipping with a first class multiplayer element."
Wolfenstein: Enemy Territory
The game uses a modified "Return to Castle Wolfenstein" engine, itself being a heavily modified id Tech 3 engine, which has been open source since 2005. As of the first day of the August 2010 QuakeCon, the entire source code was released under the GNU General Public License v3.