The MacHack developers conference always used to roll around about this time of year, and even though the conference is no more, the itch to create utterly cool but completely useless hacks is back in season. I don't have a recent PowerBook, iBook, MacBook, or MacBook Pro with which to test the Sudden Motion Sensor hacks that have been appearing, but watching the videos is probably safer anyway.
Anthony Maddox's MacSaber uses the information from the Sudden Motion Sensor to cause Macs waved in the air to make Star Wars light saber noises, enabling silly looking battles between geeks wielding expensive PowerBooks.
For another take on how to abuse the Sudden Motion Sensor, check out Erling Ellingsen's SmackBook Pro hack, which ties in with a virtual desktop utility to enable the user to thwack the Mac on the side to switch to and from different desktops.
These hacks raise all sort of other uses for the Sudden Motion Sensor that would be satisfying, if undoubtedly bad for the Mac: