Guerilla marketing with a difference: Ads that take advantage of our dirty streets.
Not only is it cheap as you don't actually buy any space, no environmentally unfriendly inks are used. Depending on the execution, it can also be more eye-catching than your standard poster ad.
Finally, while fly-posting and wall markings / chalkings can theoretically get brands into hot water, you'd imagine any over zealous local official would have less of a leg to stand on here given it's nothing more than a water mark in the urban grime (and so as much of a social statement as an ad).
Depending how tardy your local council is in keeping the streets clean, Green Graffiti's creations apparently last for up to six months.
Pictured: Before and after shots for Dutch brand 5010, and Smart car advertising.