"Forward This message to get a free .... !"

Many hoaxes encourage their victims to forward a message to as many people as possible in order to get a reward for themselves or on the behalf of some charity. The hoax claims that if enough copies of the message get sent then something good will happen. Alternatively, some messages claim that unless enough messages are sent, than something bad will happen. Popular variants of this are currently centered on The Gap, Disney, IBM, Microsoft, and Bill Gates.

The thing to know is that  there is no way for anyone to  count the number of copies of an email in circulation on the internet, nor to count the number of times something has been forwarded. In order to actually do this, not only would you have to run a program that would "open" and examine the contents of many millions of email messages all over the world, the program would have to trace the path of each message. This would be an incredible technological challenge with no real return on the investment.

More examples here.