A brushbot is a circuit with a vibrating motor on top of a brush such as a nailbrush or toothbrush.
The kits are a bit pricey ($15-20), so I decided to make my own using a nailbrush, a battery holder, a switch, and a motor. I had the motors from another project but had gotten them for about $1.00 each, the battery holder was $2.00, the nailbrush, $1.00 from Target and the switch was $3.50 at Radioshack. So the total cost for my brushbot was $7.50. It could have been cheaper if I had bought the components in bulk from internet supply stores, but I had only 4 kids in the class, so Radioshack was convenient.