Sculptris is a free sculpting program made by Pixologic, the creators of ZBrush. Sculptris is simple to use, with a clean interface and intuitive tools ranging from sculpting to texture painting. It is a great point of origin for a sculpting workflow, though it lacks some of the more advanced features needed for animation and rendering. As a sculpting tool, it is primarily aimed at creating organic models, not hard surface objects. It is available for Windows and Mac.
Sculptris Website