We recently posted about a free AutoCAD alternative called BRL-CAD, which is very advanced and build by the US military in collaboration with NASA. But the application is not that easy to use by normal people, not very much into CAD applications (or the army :-) ). So for something easier to use, I suggest the following two applications:

1. FreeCAD, a free general all-purpose 3D modeling application, focused on mechanical engineering and related areas, such as other engineering specialties or architecture. FreeCAD features tools similar to Catia, SolidWorks or Solid Edge, and therefore also falls into the category of MCAD, PLM, CAx and CAE. FreeCAD works on Windows, Linux and Mac OSX.