aXbo alarm clock
Heres quite a useful gadget I think. An alarm that detects when you have had enough sleep and then sets itself off.
A wristband that you wear detects your body movements and decides when you have had the optimal amount of sleep.
One downside that I can immediatly spot is the fact that it would be useless in waking you up in time for work or school, which is when you really need to be up at a set time. Also, you wouldn't need it on weekends because you can sleep in for however long you want.