Preview is awesome for a number of reasons, and writer Dave Winer points out that it also makes a great paint program.
While you can’t create new works of art using just Preview, it is a more powerful editor than it looks at a glance. Just open up any image in Preview, click the toolbox icon, and you’ll get a bunch of editing tools. You can easily make backgrounds transparent, annotate images, draw whatever you want as an image, and more. Preview doesn’t have the charm of MS Paint, but it does a bit more than you’d think.
A hidden Mac paint program | Scripting News
via Lifehacker
Access Your Mac’s Hidden Paint Program In Preview