I have used the GIMP in place of Photoshop on my old Powerbook (and on Linux) for some time now. I'm not really a power user of image manipulation tools, so the GIMP is more than powerful enough for my requirements. The problem with the GIMP on OS X is that it needs to run under X, so it doesn't play with Aqua / Quartz Extreme and you need to be a bit of a techie to get it all together. Fine for me, but not for most, I reckon.
Enter Seashore, which is a project based on the GIMP, but written in Cocoa, so it behaves like a normal OS X application - no X windows required. I'll be installing it shortly........