The following resources are useful for a GIMP user, even if some of them apply to older versions.
The GIMP User Group offers an informative Web site at http://gug.sunsite.dk.
http://www.gimp.org is the official home page of The GIMP.
Grokking the GIMP by Carey Bunks is an excellent book based on an older GIMP version. Although some aspects of the program have changed, it can provide excellent guidance for image manipulation. An online version is available at http://gimp-savvy.com/BOOK/.