FreeCAD is a parametric 3D modeler. Parametric modeling allows you to easily modify your design by going back into your model history and changing its parameters. FreeCAD is open source (LGPL license) and completely modular, allowing for very advanced extension and customization.
FreeCAD is multiplatfom, and reads and writes many open file formats such as STEP, IGES, STL and others.
Available: http://www.freecadweb.org/