What are some of the craziest growth hacks you've heard of?

To me it'll always be the AirBnB's integration with craigslist. When AirBnB was new, they let their users cross-list on craigslist with just one submit. One click and a user could list on both AirBnB and Craigslist. When anyone searched Craigslist for a rental, they'd see AirBnB listings. Piggybacking on craigslist's popularity was nothing less than a genius stroke. Do you have any crazy stories?