ZBrush seemed to fit the bill pretty well. You start with a ball of virtual clay, and gradually sculpt it by shoving it around with your mouse or tablet. The full program is insanely expensive, but there is a trial version available. Before I got as far as downloading that, though, I noticed that the developers of ZBrush also offer another program called Sculptris, which is targeted at beginners. AND IT IS TOTALLY FREE.
It's insane. Everyone should download this program. It is SUCH a fun toy, and really easy to get the hang of. I think it's easier than drawing in some ways, because all the tricky shading is automatically done for me. Yay! I cranked out these two heads in one evening. They're still pretty rough, but I'm very pleased with how they're developing.
|A raisiny old man!|
|Breckendy, sans hair.|
It's an exciting adventure into the 3rd dimension. I'm eager to explore further...