NaroCAD is a fully fledged and extensible 3D parametric modeling CAD application. It is based on OpenCascade.
This software looks VERY promising, but I'm sorry, if I can't draw a simple rectangle within 30 seconds, there's a serious problem with the user interface. :( I tried this on the latest stable version 1.6.0. I even followed the tutorial which popped up on the side, but didn't work. I'm running Windows 7 x64.