I stumbled upon Sculptris a few weeks ago, and it looks pretty promising. It's very unconventional, but it produces high quality models and is very user friendly; I've never modeled something before and I like it. There is also a not quite finished polygon reducer, which means you can use your models in games.
Why isn't it open source? Open source products often go crazy and unusable. Example: Blender. *shivers*

We know ;-)
I tried it after it was mentioned in that thread, and while it's obviously very powerful, it's somewhat unintuitive, and quite bug-ridden. I expect it'll get much better in time though.