Quake3 Mod Source
There is a "How to make a mod" text file included which shows the very basics of building the source for debugging ( Win32, Visual C++ ) and building the vms for distributing mods. It also shows steps to get your mod to run with the engine.
PLEASE NOTE: The game source MUST BE INSTALLED in quake3 off of whatever drive you like. This is so the batch files and the lcc compiler can find all the right stuff for building vm's. The tool source and video compressor will be out next week. This will include source q3data, q3map, q3asm, q3radiant, our version of lcc and the executable for the video compressor.
Here are a few more mirrors that you can download the source from: shugashack.com , ftp.cdrom.com , liquid.lithium.com , www.quakecity.net , www.ve3d.net