Blender

Blender is a 3D modeling, animation, and rendering program with capabilities bordering on the professional (and certainly with features I only wish I had in my Lightwave 7/8 days) that is, nevertheless, absolutely free.

Yes, free. And it runs on a Mac, on Windows, and in Linux. 

Will you get a big career in 3D animation using Blender? Probably not, even if it was used for animatics in Spiderman2. What it will let you do, though, is learn the basics of building models, creating surfaces and textures, and animating your creations without spending thousands of dollars for Modo, Lightwave, 3DSMax, or Maya. I think the “free” price tag (compared to the thousands Lightwave cost when I bought it) and its inherent capabilities combines with the opportunity learn the thought process of 3D modeling more than make up for the learning curve of a new set of tools once you decide that you wish to pursue it further.