If you're looking for an inexpensive alternative to Photoshop I hear this is a good one.

A tutorial that manages to be simple yet advanced on a subject that is surprisingly complicated. Personally I tend to let Lightroom do the dirty work these days but, and it's a big but, it's important to understand what's happening here. <i>via <a href="http://www.mattmurtaghphotography.com/?p=30">Matt Murtagh</a></i>

Extensive testing of Acorn, Drawit and Pixelmator, the latter of which I've been using with occasionally sketchy results. Might give Drawit a go.