The WWF advertising peeps are on fire at the moment with several awesome advertisements. Each urges us to protect the environment in its own unique way.
My personal favourite is the human fish head - it really scares me and is an image I will certainly never forget. So be default a great advertisement. I also love the eye chart with the birds, once again a simple image that gets the message across. And the ice cream iceberg one is fabulous, and could even give you the chills (sorry).