The GIMP:The GNU Image Manipulation Program, for X Windows systems. Official site, including download area, documentation, FAQs, and a plugin registry.
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.
Everybody loves The GIMP: GIMP guru and plug-in writer. Keeper of the nexus of GIMP links and information.
Gimp PDB Documentation: Gimp Procedural Database Documentation created by gimpdoc.
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.
GIMPwin Information: A collection of various GIMPwin-related stuff. Currently mainly a HOWTO-gimpwin-gtk-themes is provided.
GimpWin installer: Provides installations packages for GIMP on Windows.
GTK+ for Windows: Uses Tor's libraries to create installation packages of GTK.
GTK-Wimp: "Windows impersonator" is a GTK theme that blends well into the Windows desktop environment.
Jürgen's GIMP Stuff: Submit ideas for something to be added to the GIMP. The wish list is a list of ideas for hacks/modifications/improvements of the GIMP.
Jon Peterson: An interface critique of The Gimp
Planet Gimp: Collection of Gimp artwork and a tutorial.
Sunny Spot: Includes a set of brushes, plugins and several scripts for Gimp users.
WinGIMP: Win32-related GIMP information.
Zach's GIMP Stuff: Some collected information, tutorials, news and a FAQ forum.