As you may know, we are pretty hardcore about using open source software at Fablab Amsterdam. FreeCAD is a parametric 3D modeler made primarily to design real-life objects of any size. FreeCAD is open-source and highly customizable, scriptable and extensible.
I have made a small videos series about how to use FreeCAD for 3D modeling, which I have shared via our 'Makers of Waag Society' video channel on Vimeo. The first one is embedded below. This basic tutorial will teach you how to use the main commands of FreeCAD to design a knob. Please feel free to share them all online!