I don't know if it's the new economy driving people away from pricey alternatives, or if Adobe lost the asstroturf war and now people understand that Gimp is a viable image editor. All I know is that Gimp just seems to be a buzz topic lately. Since I have searches pouring into my site to find the five or six little Gimp tutorials I have, here is a whole list of resources from my personal bookmark list, covering everything from beginners to intermediate to expert advice:
Learn the Gimp:
Meet the Gimp! - The Gimp podcast, with video tutorials!
Gimp.org - The official Gimp site.
MMMaybe.Gimp.org - Gimp.org's own small list of tutorials, organized by category.
Carol's site at Gimp.org - A collection of Gimp tutorials and resources.
"Grokking the Gimp" - A free website eBook! May be getting old, but it still gets the beginner quite far. Also check out the recommended reading list, with actual bookstore-books you can buy.
GimpTutorials.com - A collection of user-submitted Gimp articles.
Pixel2Life's tutorials - Pixel2Life has tutorials sorted into categories.
Gimp Guru - They have tutorials, a mailing list, and an "ask the guru" feature! Bless them!
Gimpology - User-submitted tutorials.
ANG Pilipino Gimp - Collection of best tutorials from 2008.
My own humble little collection of Gimp tutorials:
- Beard/Hair Tutorial
- Metallic Text
- Pebbles
- Popout Photo
- Mirrored Ball on Checkered Plane
- Tracing a Cartoon Figure (with some help from Inkscape)
- Fast Flaming Text
Gimp Resources:
GIMP Plugin Registry - The official Gimp Plug-in Registry (How could I forget this?)
Graphics Muse - A set of plugins, brushes, and patterns for the Gimp.
SEO-Expert resource list - Another list very much like the one you're looking at now.
TechZillo - List of plug-ins.
Noupe brush set - 1000 free Gimp brush sets!
FX Foundry - A project packaging many plug-in scripts for the Gimp.
20 GIMP plugins for Photoshop-like functionality - This list of plugins focuses on replicating Ph*t*sh*p features. *Cough* *gag*. Well, if you want that, there you go.
Gimp Communities:
GimpTalk - A Gimp user community forum.
GimpDome - A user community with tutorials, weekly challenges.
If anybody else knows an up-to-date resource that I missed, particularly user communities, post the link in the comments and I'll check it out (and add it if it's viable).

Comments:
You will find about 800 tutorials about GIMP. The filters are also explained with images and parameters
http://linuxgraphicsusers.com/forum/
Cheers
a great selection of addon filters for GIMP, add things like lomography effect, cross processing effect etc.
Well worth the download and installation.