This one should prove interesting to MichaelG if no-ne else!
openflexure.org/

Also a block stage:

It seems little more than a Raspberry Pi and camera are needed to get started and despite claims elsewhere of 'lab standard microscope for $18' I think that you need to add you need to fit a high quality objective that will cost a bit more than $18 to get the best from it (although if you already have an objective..)
I might print the stage and see if I can get it to work with my USB scope.
Neil