We swore off microscopes after getting one for our 10th birthday and seeing just how disgusting our skin was, but this cheap USB microscope seems to be kinda neat. Carlosb.tv shows you that you can make your own microscope by dismantling a cheap USB webcam, placing something really dirty on it, turning out the lights and shining a flashlight on the result.
Not to spoil the ending of the movie for you (the webcam is actually a ghost), but the first commenter says you can even use this to detect sperm. Not that it comes, you know, in undetectable quantities or anything. If you see some, chances are that there's enough to tell what it is. [Metacafe]