"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."