Jitspoe's Joint
Tips and tutorials on Quake 2 editing, along with a few downloads.
KrAy-ZeE's Modeling Guide
Guide to making models and skins. FAQ available.
Q2Java
Information on coding mods in Java. Documentation and downloads.
QERadiant
Free Quake II map editor that is based on id's own QE4. Includes tutorials and tips.
Quake 2 Scene Builder
View models and skins, and pose them for scenes. Screenshots and gallery.
Quake II Compile Center
Software launcher for the level compiling tools. Screenshots, feature listing.
QuakeLab 2
Information on getting started in Quake2 level design, as well as advanced topics and downloadable examples.