Those who know me know how much I like the iSeismo app for the iPhone. The phone as a built-in accelerometer and the iSeismo app uses it to show the movement of the phone in three dimensions. The app show three graphs (seismographs), the first two show horizontal motions and the third vertical motion. So, if you put the phone on the table and hit the table the third line should jump up and down.
You can also export the data from the phone (or iPod Touch), and since the phones’ times should be synchronized pretty well, there should be a way to use two phones to triangulate the location of an impact, say on the floor in a room, in the same way that seismologists use seismographs to locate earthquakes. That would make a great demo if it was easy enough to do.
Update: iSeismo can also be used as a heartbeat monitor.