Gimp.app is a
self-contained application bundle of the GNU Image Manipulation Program for OS X. I’ve seen it mentioned before but I finally downloaded it and experimented a little. It required also downloading and installing X11 – URL included in the download – but that was all very easy. The downloads are large and not dial-up fodder though.
The whole thing is very impressive. It closely resembles Photoshop, though it quite possibly has a few more features. It’s free too. If you have cable internet and want to manipulate images then certainly give GIMP a try before spending vast amounts on Photoshop.