What a day for coolness! First the coloured bubbles, then this... It's the lastest in "grafitti technology". They're called LED "Throwies" and are touted as "...an inexpensive way to add colour to any ferromagnetic surface in your neighborhood". A Throwie consists of a lithium battery, a 10mm LED and a magnet - all taped together. "Throw it up high and in quantity to impress your friends and city officials".
Watch the video... very cool effect. Watch the other videos on that page as well. There's some other cool ones (Like Throwies II).