Sculptris

Pixologic (the makers of ZBrush) have released a free 3D modelling program called Sculptris. It starts you off with a sphere that you can manipulate. It also has a simple paint feature that lets you paint right onto the model you create (as well as being able to export to ZBrush). A quick 20-minute play (so don't judge) resulted in the following:

The list of system requirements are here:

http://www.pixologic.com/sculptris/requirements/

The gallery is here:

http://www.pixologic.com/sculptris/gallery/

And the features list is here:

http://www.pixologic.com/sculptris/features/

There downloads link on the site, I couldn't post it here as it launches a pop-up which asks you to choose your OS.

Anyway, it's free so have a play! :)