Granted, The Simpsons is a well established brand. But here lies an example of something with a strong propensity to spread virally.
As an on line promotional vehicle for the new Simpsons movie, create your own Simpsons avatar. Click on the donut.
It makes this website really sticky, the really clever thing is that your avatar then appears randomly through the website. Very cool. I just spent quite an amount of time making up avatars for all my friends and family members.
How did I find out? Word of mouth, multiple times.