Almost nine years ago, I played Return to Castle Wolfenstein, a classic 3D shoot-em-up with Nazis and supernatural horrors and completed it. I also played the multi-player online. Particularly memorable were the flame thrower and the big climactic battle against the resurrected Knight.
ID Software has released the source code to the game and the add-on, ie along with Wolfenstein - Enemy Territory, Return to Castle Wolfenstein single player and Return to Castle Wolfenstein multiplayer under the GPL license. These aren't just for Windows, but include Unix and MacOSX code as well.
Note these only include the game source code, not the graphics. You need the full games for those. I was going to suggest checking to see if websites like Gog.com had the full game but unfortunately they appear to be out of business. That's very sad.
PS, note this is to the FTP download site. Internet Explorer and Firefox can both handle FTP sites.The FTP folder also contains the source code for other ID games such as Quake 1 and 2 and Doom.
- Link to C Games Code Library