Need to do some guerrilla street advertising? What you need is the ChalkJET 9000, an ink jet printer on wheels.
Using two Arduino Duemilanoves for the brains, this little cart features eight cans of spray chalk which can be individually actuated. Small solenoids pull down on levers in order to spray the cans. Encoders on the wheels of the cart keep track of the spacing in between each pixel as the cart gets dragged along.
A small LCD mounted on the handle allows you to select which text you would like to print, but it doesn’t look like manual entry of new words is possible — You’ll need to load up a library while connected to a computer before hitting the streets!