I have started using FreeCAD to design parts for my models. I will be making some of these available for download from the site.
One of the things that I found useful was to create parts objects for things I use in my models (servos, prop guards etc.) I can import them into the FreeCAD drawing of the part I am designing to see how they fit. I will be making these available for download also. They are available as .step files so I guess they could be used in a number of 3D CAD packages as well as FreeCAD.
So far there is a SG90 9 gram servo and a DJI Phantom 2 prop guard. You can check them out in the FreeCAD section.