QuakeCon festivities, id's John Carmack revealed something that should excite the modding community: source code. Authors can now access source code for both Splash Damage's multiplayer-focused Wolfenstein: Enemy Territory and 2001's series reboot, Return to Castle Wolfenstein, through id's FTP, which we've linked below.
Both games' sources are made available through the GNU General Public License agreement, which states that the code is available for both free and commercial use, so long as the individual using the code credits id Software and doesn't try to claim it as their own. If you want to tinker with either game, check out the convenient Big Download link below. Happy modding!