Freecad is an amazing tool. It is based on the open source CAD kernel OpenCascade: http://www.opencascade.com/
If you want to do CAD directly you can use OpenCascade directly or if you can use its various wrappers. The Python OCC wrapper is quite nice: http://www.pythonocc.org/
We use both Python OCC and FreeCAD to help with handling CAD data import/export in http://Clara.io
Although if you are interested in create CAD online, I recommend http://onshape.com
If you want to do CAD directly you can use OpenCascade directly or if you can use its various wrappers. The Python OCC wrapper is quite nice: http://www.pythonocc.org/
We use both Python OCC and FreeCAD to help with handling CAD data import/export in http://Clara.io
Although if you are interested in create CAD online, I recommend http://onshape.com