Acorn, the image editor for humans.

Acorn is an image editor built for the Mac with simplicity in mind. Fast, easy, and fluid, Acorn provides the tools you need to alter and enhance your images, without any overhead. Acorn feels right, it won't drain your bank account, and you don't need a Ph.D. in computer graphics to use it.

I've been using this almost all day and really like that the key commands are VERY similar to Photoshop - all of the key ones I use all the time without thinking are the same so far.