> Well.. As someone that can't afford the "commercial" ones,

I don't think many individuals can, or rather any would want to spend 
their own money on it. I certainly can't!

> all I know
> about them is that they sometimes store things in ways that do not
> convert, and every damn one of them seems to have some proprietary
> format, making it impossible to import.

There are 3 types of data structures most CAD software can load/save. 
The 1st is the native data to that piece of software, which is normally 
not compatible with other CAD software unless you have some expensive 
conversion software (it does exist). This type of data stores all the 
definitions of each step of the "building" process to get to the final 
model, all parameters, equations etc. It's a bit like having the SDL for 
a POV scene, once you have this you can easily make tweaks to the 
object. All CAD software has some form of API or scripting language to 
build up its native data, POV could make use of this if you wanted to 
convert POV SDL to CAD data for meshing or real-time display.

The 2nd type can be exported and imported by all CAD software and STEP 
or IGES is the common format used. STEP files hold mathematical 
definitions of all the geometry, but no information about how it was 
constructed. This type of data would be perfect for sending files from 
CAD to POV to be rendered, as no meshes are involved so quality would be 
perfect. In theory you could write a converter from STEP to SDL, but 
there isn't one I've found yet :-(

The 3rd type is a triangle mesh, all CAD software can import and export 
this, and is most often used to export to other software to be rendered. 
Obviously as it is an approximation to the true design you need to 
carefully choose the mesh settings in the CAD software when you export.

> Damn 3D apps are like that too. There is this site, its all engineer
> types, and the objects are free, though there may be restrictions on
> some usage:
>
> http://grabcad.com

Useful link, thanks. Your only real bet for POV use is the STL files, 
they are just triangle meshes and there is a converter stl2pov. For the 
STEP files you'll probably need something paid, as it's not trivial to 
mesh a STEP file...

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