The bristlebot is nothing more then a toothbrush, a vibrating motor and a battery. The vibrations make the bot move around.
These are fun to make, and quite cheap.
update: I added a snail version of the bristlebot as well, this holder is made for L1154 batteries which are 1,5 volts. This way the bristlebot moves slower, but doesn't fall over as much.
<3 those googly eyes!