Wow... have I got a program for you -- Its called Alchemy and it's Free! (http://al.chemy.org/ Still early in design)
Rather than blunder over my own words to describe the program, here is a snipit from their website:
One of the big names being pushed along with this program is famed Andrew Jones for his very unique and creative style. He stresses the importance of accepting all artistic applications as a means to an end. Not that Photoshop is better than Painter or that Mudbox is better than Zbrush. They can all have a creative hand in the final product/object/piece. An interesting fellow indeed.
After playing around with Alchemy, it would be easy to fall into the "Andrew Jones effect" but if you stick to what you know, this program still generates really fascinating quirky/happy accidents as you draw. It's almost the digital equivalent to building up ink spatters till forms and shapes are discovered. The silhouette thumb nailing approach is really fast and fun with this application.
I created a few design in Alchemy then to Photoshop and then finally in Painter. Really easy, really enjoyable work flow and I could see spending many hours doing this method. Or just to get away from all the illustrative rules and make some abstract work.
Check out Andrew's demo of Alchemy Below: