Corel Painter X. I already bought the Wacom Bamboo.
I am a graphics programmer by trade, but a couple months ago I got very interested in drawing and painting with acrylics. The articles on color theory from an artist's perspective is so different than ones for graphics researchers, so the new perspective has already been rewarding (for example: four page article on light).
I used up a 30 day trial of Photoshop, but a standalone copy is outrageously expensive for someone doing this as a side hobby. GIMP is free and ok, but I can't be bothered with hunting down brushes or making my own. The Wacom Bamboo came with Painter essentials, but I would prefer to pay money for something better. I downloaded a trial of Painter 11 and came to appreciate its brush system. It has about a thousand brushes and each is so different, including the ones that do nothing but blend colors on the canvas. I really like its color picking pyramid and its price is reasonable. I downgraded to a trial of version X which I will buy instead - it has pretty much everything I need from 11 and is less expensive. Version 12 is probably going to be out soon anyways.
My first game will be a jigsaw puzzle game with a Christmas theme on iPhone, iPad, iPod Touch, you name it. I reserved some time to do one picture every two days. So far I have these:
Currently I use mostly the chunky oil pastel brush in Painter with a grainy blender + round blender combo to blend colors. It works well. If I get bored then I could always switch to water colors or something.
I am creating four more pictures because I figure eight Christmas jigsaw puzzles in a single app should be enough.