I was just watching "Attack of the Show!" and the had a segment in it about make magizine. They were showing off their "roomba battle bots" $@#$@#$@#$@# fighting and the last thing they had on was a new kind of graffiti. It is just an LED, button battery, and a magnet all wrapped together with tape. with these the LED lights up and you can throw them on anything metal and they will stick. They said they usually last a couple weeks. I just though this was pretty cool.
http://www.instructables.com/ex/i/7D...C001143E7E506/