Reminds me of my Web Doodle thingy
I think the coolest brushes however are the ones that use derivatives to help smooth out your lines. It's pretty cool to alternate between a pencil tool and special brushes and see what kind of a difference it makes in your quality.
I've seen a game where you have to draw out stuff in order to help your "lemming". It was pretty fun, although they used a standard pencil algorithm so your drawings looked terrible. It would be cool to see a few more games like that, but with better brush algorithms.