You may get some more complet info from others here, too. The issue
you ask about is not “which cad program” at all. Any Cad program
will do. This issue is “how to integrate it into a web site”. That’s
something else entirely. The simple, painless and easy way to do
that is to save your work as a jpeg, and then it’s just a picture.
Otherwise, you need to get into VRML, or use.dxf files and then
Autodesk has a free DXF viewer (or did), and everyone will have to
download it to view the pieces. To get a 3d piece in a web site that
can be rotated and all is the hard part. Or you can make a Flash
movie of it - that’s a big deal to do for a whole catalog, but one
piec isn’t so hard.