I use many, but I use Rhino 3D the most, and love it. It seems that I when someone sends me a file Its just easier to work on it with Rhino, and then send it in their format, something Rhino does natively.
The learning curve is rather low, its very intuitive and it performs many tasks that no other SINGLE software does; Such as, convertions between most formats, it provides the ability to export your work in common rapid-prototyping languages, it allows for homemade 3d scanners*
to be used/accepted, it will allow for a large design pipeline with many people working on the models at the same time, Where autocad sucks your puter to a crawl Rhino seems to skip along like a 2kb exe... the list goes on. I hate to sound like some kind of Rhino Salesman, however This program is definitely worth your consideration. It just does so much and its soo inexpensive. The absolute most awesome part is how you spend most of your time designing and being creative... Not learning stupid CAD software.
Its not that expensive... and it gives you a trial version that you can Down load for free. and there other ways of getting it, if you want to play with it or do non-profit projects
, PM me if you want to know. but if you want to get it for free and make money off it, I suggest to buy it, otherwise they will die off, and I will be very sad, nai I say angry. http://www.rhino3d.com/
..X..* I usually just make prototype parts using modeling clay, stick it in the freezer and scann it. Makes quick work, quicker. and if you are into Reverse engineering, Again a Must get... they make it soo easy to do.