I’ve been using Gimp on a Mac. Not as good as Photoshop but you can’t beat the price.
Used Gimp, it’s not as user-friendly as Photoshop (ok, user-hostile is a better term). But for 99% of what I do, Google’s free photo-editing Picassa is the best deal I have seen. The only thing this free program doesn’t do, is erase images from a picture. I expect a new version will enable that feature too. If you haven’t tried it, give it a whirl.