Here's a great article at Evil Mad Scientist Laboratories showing you how to make a skittering robot out of a toothbrush, a little electric motor, and a watch battery. Nice little film there, too
We did something similar to this on the last series of The Big Bang, using scrubbing brushes and the cheapest electric toothbrushes we could find. You glue a stick to the toothbrush head so it waggles back-and-forth, then rubber-band the whole toothbrush to the scrubbing brush. On a smooth surface the resulting contraption will wander around – ours mostly went backwards, as I recall.
You'll find more about these – and a bunch of other great ideas – in Neil Downie's terrific book 'Vacuum Bazookas, Electric Rainbow Jelly, and 27 other Saturday science projects' (we're not kidding, that really is the title).