The latest Apple laptops include a motion sensor to protect your hard drive if the Mac drops. But a handful of free programs put the sensor to more creative work. BumpTunes lets you change your iTunes track by lightly tapping your screen, and Dashboard Level gives you an onscreen level. Perhaps most useful: iAlertU will set off a screaming alarm if anyone tries to move your Mac.
Click here for a handy collection of motion-sensing Mac apps.