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.
Check the video for footage of a few dozen people plastering a Chelsea building with hundreds of electric lights, then head over to Instructables for a complete step-by-step guide to making your own, for under $1.00 apiece. Looks like a hell of a lot of fun, and the potential further uses is pretty remarkable. Check it out at Graffiti Research Lab.