SeisMac accesses your laptop's Sudden Motion Sensor in order to display real-time, three-axis acceleration graphs.
The resizable, real-time scrolling display shows an enormous amount of acceleration information. Place your laptop on a table and see the seismic waves from tapping your toe on the floor. Lay your laptop on your chest and see your heartbeat. And of course, if there is a real earthquake, SeisMac will be displaying full seismic information while you drop, cover and hold-on.
It's freeware and based on the open source library SMSLib.