LED Throwies
“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.” Developed by the Graffiti Research Lab, LED Throwies seem to be incredibly easy and cheap to make.
Instructions on how to put them together can be found at Instructables, and a great video of a LED Throwie gathering in action can be found at the G.R.L site, guaranteed to make you want to go and throw some throwies.
