The latest generation of PowerBooks have an accelerometer whose purpose is park the hard drive's heads in the case of sudden acceleration (which typically happens when you drop it). Although this feature isn't new to laptops -- some IBM laptops had this feature prior to Apple's incorporation of it -- it took some PowerBook hackers to really take advantage of it.
Amit Singh over at Kernelthread.com has a pretty complete page describing the accelerometer, which Apple calls the AMS, short for "Apple Motion Sensor". Even better, he's been able to tap into it and write applications that use the AMS' data!
The AMS Visualizer is an app that uses OpenGL to render a 15" PowerBook hanging in space. The image in the window reflects the PowerBook's orientation: tilt the PowerBook to the left, and the image in the app also tilts left; tip it back, and you're treated to an underside view onscreen.
Stable Window is an app that draws a window that stays level with respect to the ground. If you tilt your Powerbook in one direction, the app tilts the window an equal amount in the opposite direction.
Someone should bring an AMS-equipped PowerBook to the Santa Cruz Mystery Spot and try this app out!
The Perturbed Desktop is the aforementioned Stable Window taken to a silly extreme: it tilts all the windows on your desktop based on a combination of factors including the orientation of your PowerBook.
Matt Webb at Interconnected.org took the approach even further and wrote bumptunes.py, a Python script that uses the accelerometer to control iTunes. This application lets you jump to the next track by tilting the machine backwards and to the previous track by tilting it forwards. Don't like the current song? Give your PowerBook a light whack and you'll skip to the next one.
(This is a wonderful embodiment of Joey's Rule of the Percussive Maintenance of Machinery: "Hitting it once is maintenance; hitting it twice is abuse.")
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