Boing Boing buddy Eli the Bearded sez:
Some Apple Powerbooks have a motion sensor inside that allows the computer to detect how it is being moved. This is useful for, eg, emergency harddrive head parking when a sudden fall is detected. Amit Singh, while preparing for a book on OS X internals, has written several programs that use the motion sensor for other purposes. This page mentions those and then goes into great detail about a device driver to use the motion of the computer as a substitute mouse.