The source code is available on ODSN and all new code is licensed under a modified Sleepycat license. Here's why you should kill your time with Zandronum, versus vanilla Doom:
Zandronum brings classic Doom into the 21st century, maintaining the essence of what has made Doom great for so many years and, at the same time, adding new features to modernize it, creating a fresh, fun new experience. Zandronum is a multiplayer oriented port, based off Skulltag, for Doom and Doom II by id Software. Welcome to Zandronum! Leading the way in newschool multiplayer Doom online.
