Logging earthquake data with an accelerometer.
Back in September, there were some very significant earthquakes in Christchurch, New Zealand. Fortunately (and truly miraculously) no one was killed. One of SparkFun's good friends, Philip Lindsay, lives in Christchurch. Like any good electronics enthusiast, in the midst of disaster, Philip saw an opportunity.
As is typical, after the initial earthquake there were thousands (literally) of aftershocks. Philip decided to build an apparatus that used an accelerometer and an Arduino to try to get a reading of the aftershocks' magnitudes. The results were definitely interesting, if not exactly what Philip had in mind! You can read all about Philip's project and results in his tutorial write-up.