Free Game Programming Libraries and Source Code Tons of links. Includes Wolf3D and Quake Sources at the bottom as well as countless programs, etc.
3DRad Free Game Maker Learn how to make a 3D Game Fast!

DOOM/id Software :: Classic Doom and Wolfenstein Modding
Doom World The definitive source of DOOM stuff! Every sunday the /newstuff chronicles updates
The Doom Wikia All of the information on DOOM that you could possibly need
ZDoom Website This is the source port that I try to use for all DOOM modifications
Skull Tag Here is a source port designed to include all new games using DOOM
DOOM Wad Station don't really know much ... really popular, but Spanish! A large site dedicated to all things DOOM II
The Page of Doom hosted at doomworld // more things DOOM!
PR Boom @ Here is another source port for DOOM
Paul Corfiatis's Doom Wads This guy seems to be on top of his game
The Wolfenstein 3D Dome pure dedication since 1999 (hundreds of full mods)