Ok...I can now say that OS X has 3 excellent cheap graphics apps...and they're all unique.
The first, we all know:
OmniGraffle 4.0
It's a powerful vector-based diagram/flowchart program that can be used almost like you would Illustrator. With it's new bezier curve feature, you can draw just about anything in it.
The second, some of us know, it used to be slow but it's gotten faster:
Pixen 2.0
Pixen 2.0 is still in (public) beta but this is an amazing pixel-based sprite/icon creation program.
The third is brand new as far as I can tell:
Rita 1.0.3
A freehand graphics app that allows an 'infinite canvas' as the author puts it...what you draw becomes vectorized and can scale to any size without showing pixels. This type of thing has existed in many high-end apps but never in a 20 dollar shareware app.
All these apps are either free (Pixen) or are pretty cheap compared to commercial alternatives.