Blender is the free open source 3D content creation suite, available for all major operating systems under the GNU General Public License.
This version is called a "Beta" because it's now for the most part feature complete. The Python API has had some extensive changes, most notably in naming conventions and in creation and access of properties.
Since Blender 2.55 beta over 440 bugs were fixed!
Windows, Mac, Linux
Product page: here