id Software - The guys who build Quake.
The Official new DOS-Win95 Quest homepage - The best Quake editor in this universe and far beyond, but i'm sure everybody says that of his/her editor :-). It's the only Quake editor that works under dos though and the only freeware (GPL) editor i know that works under win95/98.
Rust - Gamedesign.net - Lots and lots of information about Quake editing.
Welcome to PlanetQuake! - A large website with loads of information about Quake and Quake related stuff.