Alchemy is a free drawing program that works wonders for the starting sketch phase of a drawing/painting. You can find it here. It's a small download and oh so worth it. I had a blast using it (spent about 4-5 hours just doodling on it today). I am sure I will be using this a lot more for upcoming paintings.
Here is a side-by-side of the original sketch done in Alchemy and the finished product done in PS.
