Anyone even remotely paying attention to global warming knows that the melting of the polar ice cap is seriously threatening the world's polar bears. It's been one of the most publicized and earliest effects of global warming. Who doesn't love a polar bear?
The Environmental Defense Fund ran a guerilla marketing effort in New York encouraging riding the subway over driving to help stop the effects of global warming. It's a simple, but brilliant execution.
Check out Creativity for the credits.