LED Throwies: "Developed by the Graffiti Research Lab (see video) a division of the Eyebeam R&D; OpenLab, 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."

There‘s some argument on the site as to the environmental impact of these, which is understandable, and I’m not sure I'm totally comfortable with that side of these myself, but as a inspiration, these are great.