New York City is chock-full of free moving billboards.

(click image for closer look)
In NYC, delivery trucks—and everything else outdoors—quickly develop a disgusting, uniform layer of soot/scum/scuz on them. It's evidence of the atmospheric cocktail we breathe every day! And it happens to make a perfect guerrilla medium for free temporary messages. Mostly you see stupid obscenities on the truck panels/doors. But entrepreneurs with URLs and old gloves/chalk are starting to take advantage of what would otherwise be an expensive media buy. Like the guys at playstickball.com—mookish dudes who unfortunately take their stickball a mite too seriously. Too seriously for mighty mayor Mike Bloomberg, that's for sure. (thanks to Aurora Diaz for the iPhone snap, near Union Square)