IBM's Peace, Love, and Linux advertising campaign, sidewalk graffiti, observed on April 20, 2001.
Commercial speech masquerading as street art. In several cities including New York and San Francisco city officials complained. Usually corporations pay for their ad space. IBM refused to comment on whether the graffiti was illegal or not, however, they eventually pulled the ads. All the press and protest I'm sure helped make the campaign a success.
Credit: Ogilvy and Mather
Source: kino-eye on Flickr