Blender is an open source 3D modeling program that runs on Mac OS X as well as on Linux and Windows. Blender is free to download. You can support the Blender project by making a donation on the Blender site, or by buying books and DVDs via the site.
The headline links to an excellent, free tutorial created by Vicki Wenderlich. If you’ve ever thought about designing in 3D, this is a terrific place to start.