Some people over at the Graffiti Research Lab have come up with an ingeneous form of temporary tagging using LED lights affixed to magnets. They're calling them "LED Throwies." It's a pretty simple idea, and very cool to see in action.
LED Throwies are an inexpensive way to add color to any ferromagnetic surface in your neighborhood. A Throwie consists of a lithium battery, a 10mm diffused LED and a rare-earth magnet taped together. Throw it up high and in quantity to impress your friends and city officials.Link (via Digg)