I gotta admit, the MacBook Lightsabre was pretty cool, but the SmackBook is even cooler, and actually useful. This little mod utilizes the motion sensor to swap between desktops on the MacBook. The sensor was originally included in the MacBook to protect the hard drive against hard drops and falls, but the SmackBook is definitely more useful. Just don't smack the MacBook too hard. Check out all of the instructions for your own smacking over at the medallia blog. SmackBook Pro [medallia]
Holy crap. I thought the alarm I mentioned was a very cool PowerBook/MacBook hack. This SmackBook hack trumps it by being not only fucking awesome, but actually useful too.
EDIT/UPDATE: I removed the embedded Youtube video because 1) it's ugly and 2) you can just click here for it.