Rhino 4! Try it, amazing.
Seriously, for doing anything from unfolding for quick model building to full size manufactured production, a better 3d modeler for such a price is impossible to find. Not only can Rhino open almost any format you through at it, but it can write back to those formats as well. Don't have Illustrator? No Problem, open the file in Rhino, edit it, and return it as an illustrator file. Don't have solidworks? Open a solidworks file here to view and edit and return. Need to make a 3dPrint, you can use Rhino for this also. All in All, its the best choice for modeling with the familiar interface of Autocad, but with streamlined 3d and a set of tools that you would be hard pressed to find elsewhere.