I sometimes actually buy something with the intention of taking it apart and seeing how it works.
This falls into the "Hey! Thats worth a buck!" catagory.
This weekend at the "Dollar Store" I found small Sylvania "LED Closet Light". A matchbook sized, button battery powered, magnet activated, closet/drawer light with mating magnet assembly.
"How cool! Toy value alone is worth a buck!" I said out loud, causing people to move away.
Anywho, you double side tape the light to the door frame and tape the magnet to the door. Open the door and the tiny light comes ON. Where else can you find a magnetic reed switch for a mere $1?? Too bad the three button cells won't fit in a Slim Line switch, that would have been a really giant score.