I was reminded the other day about viral marketing. There is a great FREE book on the subject at http://www.sethgodin.com/ideavirus/01-getit.html
The basic concept that ideas act like viruses. Some ideas (the successful ones) catch on and spread. The book is about how to design and present ideas so they can be easily spread and hence improve the chances of them taking off.
A good recent example of this is beer commercials. Manufacturers have started creating adverts for release on the internet. If you wrap your message in something funny people will email it to each other. Jokes on the internet move very fast indeed. The adverts spread in this manner reach millions in no time, and costs the company practically nothing.