ArtRage 2 is a cool, inexpensive "natural media" painting program from Ambient Design. It's cross-platform, and it shows to some degree in its single window interface. The tool palettes, for example, aren't free floating windows, but rather part of the main window as you see above. Nonetheless, it has an incredible number of features for the price, offering different canvas types, painting tools, etc. For instance, you can move the "paint" around with a palette knife, and the texture of canvas affects how the pigment is laid down. All in all, it looks like a great value (and Macworld agrees) for those of us who don't need the power of something like Corel Painter X.
ArtRage 2 comes in two versions: a freeware version with fewer features, and the full version for only $20. It is available for download for both Mac and PC.