QuakeSRC - A valuable resource for Quake engine programming
Lazarus QUake II mod website - A great mod that adds tons of editing functionality.  Which is why it's integrated into KMQuake2.
QERadiant - Home of the QERadiant and GTKRadiant map editors
Rust - Lots of editing info for Quake 2 and several games
QuakeDev - The host of this site
id Software - For without whom there would be no Quake 2!