[georgemcbay]

I design all of my parts (mostly camera/lens accessories and quadcopter frames) in OpenSCAD.

http://www.openscad.org/

Not really a good candidate for a general-audience "easy to use" 3D design program (and thus doesn't preclude their plan to build something like that), but likely highly relevant for any HN readers who are looking for such a thing who are comfortable with code/markup and are wired to be able to pre-visualize things in their head prior to building them.

There are off-shoots like OpenJSCAD (see: http://openjscad.org/) as well, which actually give you more of a complete language. The OpenSCAD language is really simple and is more like a markup language with some limited flow-control than a real full-blown programming language -- but I kind of like it that way, and if you need to get really algorithmic with your designs you can always generate OpenSCAD "code" from other languages (of your choice) since it is all just text.