I saw lots of cool stuff at the Makers Faire, but the thing which evoked the greatest "this is a cool idea that I could do" factor was Tom Zimmerman's video microscope.
The link doesn't really describe why it's a good idea, so allow me to elucidate. Basically Tom takes an ordinary webcam (or even one of those little $20 B/W security cams), pries the lens off, and he's left with
the more or less naked sensor sitting there. He then takes a drop of fluid containing some of his desired microscopic critters (he was using little rotifers) and places it directly on the sensor. He then piped the video directly to a TV. Net result: blurry image, can't
see a darned thing. That's not too surprising, since there is no lens in the system, so nothing is in focus. Here's the magic: He then took a tube (a toilet paper tube would be about the right dimension) which is
open at one end, and has a piece of foil with a small pinhole in it covering the other end. When this is placed over the sensor: voila! you can see the critters, in sharp focus, swimming around munching on algae. If your light source isn't diffuse, you might actually see an image of the light source (just like you were using a pinhole camera), so Tom constructed a little light source using a bright blue LED which gave uniform illumination and looked great.
This is so simple, so clever, I'm just going to have to build one.
Here's some YouTube video of the result, it doesn't really do justice to the clarity of the images he was showing at the Faire: