I've tried The GIMP a few times ( under Windows ) and it's improved over each of them. It even has some compelling advantages over Photoshop, besides just the price. But the UI kind of bothers me, or I have a hard time adapting to it.

Anyway, a friend sent this list of tutorials for The GIMP, so I thought I'd share the knowledge: http://mmmaybe.gimp.org/tutorials/

If the UI bothers you and you are used to Photoshop try http://www.gimpshop.net/

I've not used it myself but it appears to do a good job.