Is There A Free Photoshop Alternative for Mac OS X?
GIMP is widely recognised as a quality free alternative to Photoshop in many regards.



...but it's not considered to be particularly successful in it's approach to embracing Mac OS X application design ;)

If you want something that isn't necessarily free but is at least inexpensive, then there are a number of other programs including Acorn and Pixelmator available in the Mac App Store.  You could always try the cut down Photoshop Elements.

For a good roundup, see this wonderful blog post from the Ask Different Community Blog for more info:

GIMP vs Photoshop vs Pixelmator – Graphics Editor Shootout