I've spent two weeks looking at different software packages for interactive 3D viewing of rings.
Its really hard to beat *.jpg files for their simplicity and for many designs they are really all you need.

But if you want 3D interactive display then here are the most interesting programs
that I found with a few comments (all programs listed FREE).

OpenRP STL Viewer
Reads, compresses, decompresses STL files.
There are no material properties with STL files so everything is displayed as a solid in one color.
On the positive side faceted stones remain faceted.
This is a very compact program and is perfect if you can live with STL display limitiations.

Cortona VRML Client Plugin for Netscape and Internet Explorer
As far as I can tell it's the only active VRML plugin left on the internet....
Stones are displayed with facets. I like the cad type interface best (right click in the window to switch the 'skin').

ModelPress Reader and Publisher
Initially I was extremely impressed with ModelPress.
ModelPress supports an impressive list of file formats and has lots of viewing options.
The smoothing function in the Reader when displaying uncompressed vrml files removes just about all stone facets.
When using the Reader to display 'published' compressed vrml files the stone facets are a bit better but not as good as with Cortona.

3D Compression Technologies 3DCT Viewer and 3DCT Share
Very similar to ModelPress. (I say this because I found ModelPress first and am comparing 3DCT to ModelPress)
Share supports an impressive list of file formats.
The 3DCT Viewer only reads files created by Share
Note - With 3DCT *.3DS files are a much better choice than *.VRML files if you want to display faceted stones in jewlery models.

When you save a Rhino model with faceted stones as a *.3ds file and then load and compress this file with Share
you get a great looking model when viewed with the 3DCT Viewer.
Faceted stones remain facted and materials (color and transparency) are supported.

3DCT programs used with *.3ds jewelry models are the best programs for internet 3D viewing I've found so far.
Compared to Cortona the Viewer download file is much smaller and you can also run the Viewer as a stand-alone-application
The 3DCY interface is much simpler and the sectioning capabilities are truely amaizing.

Rhino3D NURBS Modeler for Windows
Of course you can always view rhino files with rhino.....

What are NURBS: Non-Uniform Rational B-Splines.
Whatever.... for most of us all we really care is that this is one heck of a good program.
And yes, I expect to get some negative feedback on this.

The disadvantages for using rhino for people who just want to view and approve a jewelry design are the large
rhino download file and the complexity of rhino (it just isn't as easy to view a file in rhino as it is with 3DCT).
If you know and have rhino then this really isn't important - if not then you just don't want to go there.