The GIMP

The GIMP (GNU Image Manipulation Program) is a open source program for image editing that rivalls Adobe Photoshop in features. If you are thinking of getting Photoshop®, I suggest that you try the GIMP first, as it's totally free (and you can get the source code if you want to help develop it). If you want to run the GIMP under OS X, you will have to install Apple's X11 (if not already installed). You can do this from the system disk that came with your Mac. (or PC if you hackintoshed ;D). Download page is here.

The other Adobe program that there is a very popular open source alternative for is Adobe Illustrator. The alternative is Inkscape (http://www.inkscape.org/), which I have found to be in a way better than Illustrator (the version that I use is CS3), as when I opened an image that was 4707x4000 px in Illustrator it just wouldn't open, whereas in Inkscape the same image would work just fine on my PowerMac G5 2.5GB SDRAM. Both the GIMP and Inkscape are multiplatform (Windows, Mac OS X, Linux), and other UNIXes such as FreeBSD, and of course the source code tarball is available.