Editing

Jitspoe's Joint
Tips and tutorials on Quake 2 editing, along with a few downloads.
http://www.planetquake.com/jitspoe/

KrAy-ZeE's Modeling Guide
Guide to making models and skins. FAQ available.
http://www.planetquake.com/modeling/

QERadiant
Free Quake II map editor that is based on id's own QE4. Includes tutorials and tips.
http://www.qeradiant.com/

Q2Java
Information on coding mods in Java. Documentation and downloads.
http://www.planetquake.com/q2java/

Quake 2 Scene Builder
View models and skins, and pose them for scenes. Screenshots and gallery.
http://www.planetquake.com/qsb/

QuakeLab 2
Information on getting started in Quake2 level design, as well as advanced topics and downloadable examples.
http://www.planetquake.com/quakelab/quakelab2/

Tread 3D
Map editing program. Downloads, manual, and preview.
http://www.planetquake.com/tread/