Recently I was browsing the App Store and came across this tiny but cool application called iSeismometer. In a nutshell, this application uses iPhone's accelerometer to measure seismic data. The application is really easy to use. Just run it and put your iPhone on a table or any other surface. You can tap or slightly shake the surface and see the application in action. It actually measures and tracks seismic data.
The only downfall is that only 10 seconds of data is stored but as the developer mentions, this will be increased in the future releases. Moreover, you can submit the data to the developer website for future retrieval.
All in all, iSeismometer makes your iPhone even cooler toy and if you live in a seismically active part of the world, this iSeismometer is a must to have application. Moreover, iSeismometer is FREEWARE!
You can download it from this link: http://itunes.apple.com/WebObjects/MZStore.woa/wa/viewSoftware?id=304190739&mt=8