I will admit to a small polite amount of overwhelming obsession with Paint Tool SAI. Just started using it a few weeks ago and am in LOVE. There is a few things that I wish it did, like multitouch support for pinch, zoom and rotate but the painting tools are the best in the business. Smudge, Brush, Pencil, Watercolor, tools, switching transparency, opening and closing color selection, adjusting brush size, lasso, deselect, layers, color swap, delete and some of the more standard buttons you might find in a normal Artist Pad.
You can find the software at this link