Big billboard ads are an obvious source of inspiration for any street artist. Simply because they’re an intrinsic part of any big city street. There’s even a Poster Alteration Art group on Flickr. 
London has its Decapitator but I do like Poster Boy, a New York street artist that spices up the subway stations over there with his radical mash-ups of posters. Armed with a razor blade as his weapon of choice, he cuts up ads and re-arranges them on the spot into something funnier, more political, more thought-provoking. 

Check his Flickr stream here, films from Wooster Collective, Animal Magazine and his own YouTube channel and articles in NY Mag and the Guardian. 

Lots of attention, so answers on a Tweet: will he show up in an ad soon, sell out to galleries or is he using the media to start his own little revolution. From his interview with the Guardian:

“That’s what I want Poster Boy to be about,” he enthuses. “The idea of taking your environment into your own hands and making it what you want. As long as you’re not hurting other people, it can’t be bad.”