Pixelmator is the highest profile of a new breed of lightweight Core Image based (i.e. GPU accelerated) image editing programs to arrive on the Mac to compete with Photoshop at the low end.
Just released, Pixelmator aspires to be a serious contender in the image editing arena, but you should also look at Acorn (which as well as actually being available is pretty good too) and Iris (about which little is known).