Here is a list of useful GIMP related links:
Main page: http://www.gimp.org (which has also a few very useful tutorials)
Official documentation home page: http://www.gimp.org/docs/
Main plugin repository: http://registry.gimp.org/
Getting involved with the project: http://www.gimp.org/develop/
Development home page: http://developer.gimp.org/
Development frequently asked questions: http://developer.gimp.org/faq.html
Plugin development documentation: http://developer.gimp.org/plug-ins.html
Official user interface redesign page : http://gui.gimp.org/index.php/GIMP_UI_Redesign
User interface brainstorm: http://gimp-brainstorm.blogspot.com/
Tip of the day: http://gimp-totd.blogspot.com/
The artist's guide to GIMP effects: http://www.graphics-muse.org/artistsguide/
Video tutorials for the GIMP: http://meetthegimp.org/
GIMP user community: http://www.gimptalk.com/
Tutorials, community and contests for GIMP: http://www.gimpusers.com
Flickr GIMP group: http://www.flickr.com/groups/gimpusers/