In a surprise release, 3D Realms sends along the source code for Shadow Warrior, allowing you to tinker with their 1997 first-person shooter. Compiling the source will require Open Watcom 1.3 for Windows, and while the release is unsupported, it falls under the terms of the GPL (GNU Public License).
Download: Shadow Warrior source code v1.2
News source: 3D Gamers