Not quite a microscope... but some point and shoot digital cameras will focus through a loupe held against the lens housing (or even a magnifying glass held at the proper focal length, high res, then blown).
I don't know if this will work with any modern Digital SLRs, but an old trick for ghetto 35mm macro, reverse the lens (it really works). Depending on the lens and camera and your patience, you can get some pretty intense detail.
Both these methods have very limited in depth of field (very, very, but that can be good too), and paralax and chromatic abberation and a dozen other reasons to find a better way. But both may (and have) yield some surprising results on small objects.