I love effective viral marketing. Every once in a while somone comes up with a genius level marketing campaign. This is one of the best I have seen...

What is The Secret?
http://www.what-is-the-secret.com/

You can see it in action as it spreads around the world here...
http://what-is-the-secret.blogspot.com/2005/11/watch-world-light-up-with-subscribers.html