This project provides pre-build GIMP application bundles for Intel and PPC Macs.
Gimp on OSX is pretty good. Not as current as the Windows version, but it still gets the job done. The UI needs work, Single Window mode should be default on all systems, but other than that everything is there. To those Photoshop who can't figure out how to export as a jpg, pdf, it's in File -> Export. Next time look a little closer. If you're used to Photoshop, GIMP's tool placement might throw you off, but it's easy to get used to.
As far as free bitmap-based graphics editors go, GIMP is probably the best. It has some very powerful tools that are not even available in other programs. The main problem however is the user interface, which has always been somewhat backwards compared to every other graphics editor. It has not improved in the 2.8 release, maybe even to the contrary. Working with GIMP often gives me a feeling of having to take everything out of my toolbox just to reach that single wrench I need. For operations that take a single click in Photoshop, I have to click three times and press a key. The right window must be in focus and the editor must be in the right mode, otherwise operations either have no effect or do something unexpected. To make things worse, the current version of OS X Gimp has a few bugs in the toolbox interface that cause the tools to randomly switch to other modes. This happens quite often, and is only noticed when the tools do not behave as expected, because the only visual indication is a highlight change on a small icon. My verdict: if you don't mind wrestling a pig from time to time, GIMP is a great free alternative to expensive editors.
This version, 2.8.6p1, seems a bit faster than the previous OS X-native version of Gimp I've tried, 2.8.4 I'm fairly excited about the OS X-native Gimp project and will keep trying the updates..this is is an application I feel *everyone* who does any level of image manipulation on Mac should use and be interested in.