Devices that detect when someone is falling asleep
I've used the Zeo (http://www.myzeo.com/sleep/). It seems to work pretty well. It tracks which stage of sleep you are using a very basic EEG. There are some details on how it operates here: http://radar.oreilly.com/2009/10/how-the-zeo-sleep-device-works.html

As I said, I've used it in the past and from what I can tell, it seems to be pretty accurate, assuming you don't knock the sensor off while you're sleeping.