http://freecadweb.org/

FreeCad is a Parametric CAD system/Parametric model. Despite its disclaimers that it might not be ready for production use. I have found it very stable and reliable. It can read and write a variety of 3D and 2D CAD formats. The “parametric” aspect of it lets you get really sophisticated with the editing workflow. If you are familiar with Blender 3D, it is similar conceptually to using it's modifier stack.