A couple of years ago NY based artist and designer Ji Lee printed up 30,000 empty speech bubbles and started posted them on outdoor ads around the city. It wasn't long of-course before people were filling in the blanks, the most interesting of which he collected together as The Bubble Project. Now he's part of the New York Street Advertising takeover, a project that has just seen 126 illegal billboards throughout the city taken over and transformed by over 80 artists into their personal pieces of art. I rather like his contribution, above.
As an aside, he's also responsible for something else I rather like – the 'google me' business card.