Starbucks - The Red Cup.
Magnetically attached, mock venti Starbucks Red cups were placed on cars and hundreds of well-meaning people came to the rescue. The Red Cups were completely normal looking and were used to launch the now famous Starbucks Red Cups that holiday season. This was the basis for my philosophy of making the consumer the protagonist in your story. Starbucks kept this going a few years after we stopped working with them.
For being one of the most important ideas I’ve created, it maybe has the worst documentation. But this video summarizes the project. It became pretty famous at Starbucks and earned both a One Show Gold pencil and the Yahoo Purple Chair award for innovation in media. And this is the only place I ever mention any award for some reason.