Quote Originally Posted by AJenbo View Post
The engine is GPL and the best project spanned from it is EDuke32, it's gl reder was actually made by the original programmer, there is also a lot of other improvements and fixes so go get that if already have a copy of the game.
This is not completely correct.

Eduke32 comprises the gameplay engine released in 2003 by 3DRealms under the GPL, and BUILD, the 2.5D graphics engine developed by Ken Silverman. Sources for BUILD are available, it's not open source, but has a crippled license, there is also this FAQ on its page:

Why did you write a custom license instead of using GPL (GNU's General Public License)?
Short answer: to satisfy my ego Long answer: I know GPL is an industry standard, but I felt that it was way too long and boring for anyone to read seriously. I chose to write a nice short license that everybody could understand. This way, people unfamiliar with GPL would understand their rights.
Clearly noone explained him the power of free software, this engine could be much more popular if under GPL like the id's ones rather the its crippled license.