http://www.gimp.org/macintosh/
It's nearly equivalent to Photoshop (ok, not EXACTLY, but I've used it and I recommend it to all my friends who don't have hundreds to blow on Photoshop). It works well enough, it's much easier to use than Photoshop and nearly as powerful. Best of all it's completely free.
I actually had no idea it was for Mac as well, so thanks for making me look into it, now I can recommend it to my Apple wielding friends.
Enjoy.