id Software's lead programmer John Carmack has released the full source code of Quake, including the WinQuake, GLQuake, QuakeWorld, and GLQuakeWorld versions. The archive can be downloaded from id's FTP site (which is always pretty busy) and its primary mirror at Walnut Creek (3.2 MB), as well as 3D Action Gamers. The code has been placed under the GNU Public License right from the start (this happened with the DOOM source only much later), and the accompanying text file can be downloaded separately. Here are the initial paragraphs:

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.