JETFIREDX If you want something powerful, free, and EXTREMELY difficult to learn to use, get Blender. I would recommend Wings (Wings3D.com) for something that is easy to learn (though not at all Mac like) that is also free and has some good renderer plugins.
Cheetah3D and Silo I have found are also easy to pick up and learn. Silo is especially nice because the entire UI is able to be set up to fit your workflow. (I actually cloned Wings' interface with little work.) These aren't free however.
(Version 1.5)