32bits Textures Paks - Hi-res retextures usable with KMQuake2 and other modified Quake2 engines.
Inside QC Forums - 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. (Internet Archive)
GTKRadiant - Home of the QERadiant and GTKRadiant map editors
Rust - Lots of editing info for Quake 2 and several games. (Internet Archive)
Mark Shan.com - The host of this site.
id Software - For without whom there would be no Quake 2!