At last week’s Quakecon, John Carmack made an announcement that should made the PC modding community very happy: the source code for Wolfenstein: Enemy Territory
and Return to Castle Wolfenstein
have been released to the public.
The source code to both games is now available through the GNU General Public License agreement which allows the code to be made available for both free and commercial use. Of course, you have to credit id Software for the creation of the source code(s), you can pretty much do as you want with it. You can grab the material from a variety of web sites across the net, including directly from id Software at this link