I needed a cheap and simple 3D CAD app to rough out a design for a new ROV and decided to give Autodesk's "123D Design" a try ( http://www.123dapp.com/design
). I have to say, it is much better than I expected! The tool set is pretty minimal, but so far I've had no problem creating any surface or object I've needed to. Its import / export capabilities are pretty meager, but I've managed to work around it so far. I haven't sent any designs out for fabrication yet, so hopefully it works as well as I hope.