The GIMP

ACM Crossroads - Introduction to the Gimp
Student magazine of the Association for Computing Machinery. Crossroads has published an article about the Gimp, and has an online discussion forum related to it.
http://www.acm.org/crossroads/xrds3-4/gimp.html

Gimp PDB Documentation
Gimp Procedural Database Documentation created by gimpdoc.
http://www.goof.com/pcg/marc/pdb/index.html

GIMP-Savvy
Resources for learning the Gnu Image Manipulation Program. Includes the complete text of "Grokking the GIMP" an advanced book on practical image editing techniques.
http://gimp-savvy.com/

GIMPwin Information
A collection of various GIMPwin-related stuff. Currently mainly a HOWTO-gimpwin-gtk-themes is provided.
http://www.rixsoft.com/gimpwin/

GimpWin installer
Provides installations packages for GIMP on Windows.
http://gimp-win.sourceforge.net/

Planet Gimp
Collection of Gimp artwork and a tutorial.
http://www.skamphausen.de/cgi-bin/ska/Planet_Gimp/

Sunny Spot: GIMP
Includes a set of brushes, plugins and several scripts for Gimp users.
http://the.sunnyspot.org/gimp/