http://cellar.org/pictures/tinyrobot.jpg

This is supposed to be the tiniest useful robot built, although I don't know who certifies such things.

Aside from conquering pocket change, what's it useful for?

They don't know! They run for about 15 minutes on a battery charge. They think they could crawl through conduits to find problems, or that several could work together to disarm mines or bombs.

Maybe they could be sent into the bowels of my PC to recover all the screws I drop in there...

This robot is useless! As anyone who has watched Battlebots knows, the best robots are the combination of a wedge and an offensive weapon of some sort capable of flipping. This one would be crushed immediately!