[Osmium]

It recently got a big update, and now happily runs without X11. So it's better than it's ever been. If you've used Gimp on other platforms, I'd recommend it, but if you're just starting out I'd recommend something else.

Better (though paid) alternatives for OS X are Acorn and Pixelmator. Pixelmator is a more traditional Photoshop-like UI, while Acorn (in my opinion) is nicer, with a one-panel design that gets out of your way, while still having a host of powerful tools and scripting support (with a choice of languages: AppleScript, Python or JavaScript/JSTalk).

[devindotcom]

I can't stand pixelmator. And the last few releases have done nothing to improve it. Thanks for reminding me about the big Gimp release - I can't wait to switch it in!