Wired: "New Yorker Joshua Kinberg is a bike messenger of a different stripe. Instead of ferrying legal papers between lawyers, he uses a homemade, wireless, bicycle-mounted dot-matrix printer to spray protest messages in the street."
The printer is made out of five solenoid-triggered spray cans, loaded with washable chalk and mounted on the back of a bicycle. A laptop computer translates messages into a 5 by 5 pixel pattern, and then based on the speed of the bike sends firing commands to the solenoids. Messages appear on the street, from the back of the bicycle, like ticker tape.
It's awesome technology that today is being used for street protests, but tomorrow could be used for all manner of advertising.
Watch the video. It's amazing.