Using the internal sudden motion sensor in your MacBook and MacBook Pro, the SeisMac program displays real time, three-axis graphs of laptop movement. Leave your MacBook on your desk to let it act as a seismograph, or put it on your chest to measure your heartbeat. You could also use it as proof of how g*ddamn loud your neighbor's bass is at 2AM.
SeisMac also works with older Powerbooks and iBooks with the sudden motion sensor.
Product Page [Seismac]