Yes, this has nothing to do with Doom, but I figured that since no one has yet made good network code for Doom, some enterprising soul will be able to rip out Quake/Quakeworld's netcode and plug into Doom with hopefully kickass results. Quoting is easy and fun, so why not... here's Carmack.
This is the complete source code for winquake, glquake, quakeworld, and glquakeworld.
The projects have been tested with visual C++ 6.0, but masm is also required to build the assembly language files. It is possible to change a #define and build with only C code, but the software rendering versions lose almost half its speed. The OpenGL versions will not be effected very much. The gas2masm tool was created to allow us to use the same source for the dos, linux, and windows versions, but I don't really recommend anyone mess with the asm code.
The original dos version of Quake should also be buildable from these sources, but we didn't bother trying.
The code is all licensed under the terms of the GPL (gnu public license). You should read the entire license, but the gist of it is that you can do anything you want with the code, including sell your new version. The catch is that if you distribute new binary versions, you are required to make the entire source code available rce code available for free to everyone.
What this will mean for Doom source ports I can only imagine, but I hope it doesn't flush them down the toilet as coders run to newer code. Go get it.