Getting good files in order to make quality rapid prototypes is something that we place a very high importance upon here at ProtoCAM. We always make an effort to work with whatever types of 3D CAD files we're provided, sometimes even by running them through a couple of applications until they produce sound, solid, 3D geometry, but even that isn't a cure-all. Currently, customers can export STL files and then download and view them with the (free) SolidView/Lite, which we have instructions to download and install on our home page, but it has limited functionality. Another software vendor in our industry, Materialise, has just put out a custom (and also free) version of their Magics software called MiniMagics. As a viewer of the full version of Magics, I'm excited about this application. It allows exporting to compressed MGX files (for faster transfer of large parts), measuring of extents, viewable sections, viewing bad triangles, viewing unshared edges, measurement, and annotation of parts. I'll evaluate it and update our STL viewing web page if appropriate.

Link to download MiniMagics