The third chapter in the series, and the first with a 3D perspective (the original Duke Nukem and the sequel, Duke Nukem II, are side scrolling platform games).
This game, set sometime in the early 21st century, begins in a ravaged LA, which was overtaken by aliens while you were abducted during Duke Nukem II. Duke, upon returning to Earth, finds himself with another mess to clean up, and another alien race that needs exterminating.
In a surprise release, 3D Realms sends along the source code for Duke Nukem 3D, allowing you to tinker with their 1996 first-person shooter. Compiling the source will require Open Watcom 1.0 or Watcom C v10.0 or later, and while the release is unsupported, it falls under the terms of the GPL! GNU Public License.