Check out this video of a radio controlled BrushBot (AKA BristleBot) that YouTube user glajten sent in. Usually brushbots are made from the head of a single toothbrush and have no controls or steering: they just buzz about in random directions. glajten has built a steerable ‘bot, and he accomplished this by moving the bot’s center of gravity. A servo swings a weight to either side, making the bot to lean over, thus changing it’s direction.
I think this is a pretty useless machine, as long as you don’t use it as a cat tormenting device, or for sweeping up the floor.