Well, Photoshop and GIMP are both compatible with Mac.
The thing is with Photoshop is it's pretty much out of reach of any other image manipulation application. There's just so many features stuffed into that single program that very few can even come close to what its capable of.
However, a program exclusive to the Mac that's competable is Aperture.
Apple - Aperture
Aperture can make global image edits and print files without having to open them - something Photoshop can't do, It is also much faster at loading and importing RAWs from cameras; Adobe Camera RAW is slow as hell. Not quite as many features as Photoshop at general image manipulation, but unparalleled in photo manipulation.
I'd also recommend supplementing Aperture with iView Pro, which is a great cataloging app for the Mac.
Image, Digital archive, photo management and professional photo publishing software. Stock Photography and Multimedia database
It was acquired by Microsoft in a while back though, so this'll probably be heading over to Windows soon enough.
Btw, Adobe apparently released their own version of Aperture called Lightroom; I've never used it before so I don't know how it compares with Aperture.
Adobe - Photoshop Lightroom