Livebrush Pro - now 100% free
Pro version is now free! Links adjusted
You'll need Adobe Air: http://get.adobe.com/air/
Direct DL link for Livebrush Pro for those who avoid FB like the plague: https://db.tt/KqEuPapj (their link, not mine)
I ran across Livebrush when I took a peek into the "marketplace" for my Wacom Bamboo Dock, Googled it, and I saw there was a full program as well. It's not your average run of the mill drawing program. I'm not really sure what to classify this thing as, but it has both pixel and vector modes. They have a lite version, which is the one on the Bamboo Dock, a free version, and a pro version for $10. Works on both PC and Mac.
Flickr search showcasing images created with it: https://www.flickr.com/search?sort=relevance&text=livebrush
Tuts+ breakdown of it's features: http://vector.tutsplus.com/articles/reviews/an-introduction-to-livebrush-the-vector-motion-brush-program/
"What is Livebrush?
Livebrush is a drawing application. It employs an easy-to-use brush tool that reacts to your gesture. By combining simple motion controls with brush styles, Livebrush offers a fun and unique way to create graphics."
The 'brushes' I seem to be enjoying the most atm are ones that keep going even when your mouse stops. lol
There are also brushes that seem to work like Flame Painter, without the 'chain' graphic you can fling around like a rope.