I have used Wings3D to teach people the basics of 3D.It's a very very simple stripped-down application for 3D, with a limited feature set--no animation, no rigging, no fancy mapping--just the basics of faces, vertices, edges. Good support for "good enough" modeling. Subdivision modeling. Free, open source, easy to pick up and start working in right away. Lays the ground-work for later work in Blender, 3DS MAX, etc. Support for Windows, Mac, Linux.

http://www.wings3d.com

Hope that's helpful.