Nikon had a competition for micro-photography. The top 20 are published online. Diatoms and bugs seemed to be a common theme among the winners, complimented by man-made things like nanotubes and paper. Medicine, like cancer drugs and vitamin C, also seem to make interesting micro-photos. My favorite was definitely the crustacean.
I’ve posted this information before (not this exact same recipe, but something close to it), but if you are interested in making your own, here is a fairly easy how-to on taking microphotographs. I still haven’t tried it out on my own yet, but I intend to take it off my to-do list and put it on my wall, or somewhere else on display.