My first picks for free 3D software would be Blender and Wings3D.

http://www.blender.org/
http://www.wings3d.com/

Both are 3D modellers. Blender is more full featured while Wings3D is simpler but easy to learn. Recent versions of Wings3D even includes texture mapping, which makes it good enough for a lot of purposes. Oh yes, both are cross-platform!