Caligari recently released Truespace 7+ for free. Truespace is a powerful 3D application with a very pretty interface.
It can produce quite attractive rendering and animation. It also sports NURBS tools which are useful for product modelling.
Unfortunately, the free base package does not appear to support file formats essential for data transfer. It does not support IGES and STEP. It does however support STL, which makes rapid prototyping possible, but not engineering Mechanical CAD downstream though.
To take full advantage of Truespace, you will need to have a reasonable graphics card that supports DirectX. If the graphics card does not meet the necessary requirement, Truespace will not display for some of the viewing windows!
You can get Truespace from its website at www.caligari.com. You will need to sign up in order to download this powerful program for free.