Old mobile phones make excellent microscopes, for use in telemedicine (e.g. blood checks), water quality monitoring, etc. The cost is almost zero – apart from the phone itself, everything can be built from scrap parts.
This model here loosely follows the instructions on http://www.iflscience.com/ & http://www.instructables.com/id/10-Smartphone-to-digital-microscope-conversion/ (thanks!) and was built by Lars Yndal at the PITLab at ITU Copenhagen – http://pit.itu.dk