If you have a recent MacBook or MacBook Pro, chances are good that you have a built-in seismograph — an instrument that measures shaking. These are the things that are used to measure earthquakes.
What you have in your laptop is actually a Sudden Motion Sensor. This is a cute little device that tries to determine if your laptop has been dropped or otherwise jolted. If so, it parks the hard drive, hopefully preventing damage. That’s pretty cool on it’s own, but what’s even cooler is that there’s a way for applications to read the sensor and use it’s data.