ArtRage. Nice little freeware paint program. Free. Yes free. PC or Mac.
ArtRage is a painting package designed to provide a realistic and fun simulation of using paint on a canvas, along with pens, pencils, crayons, and other tools. ArtRage is available for both Windows and Mac OS X. It can be used with a mouse, but works better if you have a graphics tablet, or even better on a Tablet PC where ArtRage takes advantage of the unique interaction of pen and screen to produce a realistic painting feel.