Sculptris

Sculptris is a fantastic, free 3D modeling and painting tool created by Dr. Petter. The Sculptris tool set is similar to what you can find in zBrush or Mudbox – with one important difference. As you sculpt, detail is added to the mesh automatically so you can worry less about the structure of your mesh and concentrate on getting the shape you want. You can round-trip to your other 3D tools with OBJ import/export, paint directly on your object, apply materials, and create your own sculpting brushes. It even works with your pressure sensitive tablet.