OH!!! I'VE GOT YOU ALL BEAT!!! PHOTOSHOP
ELEMENTS, BITCH!!! That's right, free Photoshop!
Meh... it gets the job done.
I also use Flash MX (thank the good Lord for eBay). It's lots of fun.
Alias SketchBook Pro (which I may or may not have got for free... SHHH!) kicks the crap out of any sketching program if you have a Wacom tablet. It uses the same kinda thing as Flash (vector?), which means that every line you draw is exactly how you draw it... instead of being made out of pixels like in MS Paint.