Here's a quick one for those of you with access to MacBook or MacBook Pro portables. (And as Michael Flynn astutely pointed out in the Comments Section, some older iBooks and PowerBooks too!)
SeisMac 2.0 is a freeware program created with support from the National Science Foundation through the Incorporated Research Institutions for Seismology.
The program takes advantage of the Sudden Motion Sensor inside MacBook or MacBook Pro portable computers to create a functioning seismograph.
You can download the program via this website