Viral Marketing is free, word-of-mouth advertising. It’s important to provide an incentive that encourages others to spread the word about your product, quickly and inexpensively with little effort on your part. This creates potentially exponential growth in the message's visibility and effect.
Hotmail is an example of successful viral marketing. The company started offering free e-mail accounts, with a link back to Hot Mail at the end of every e-mail message sent by one of its customers. That spread the word about the service like a virus, with every customer a virtual "carrier" of the message.
These viral marketing strategies will help enhance your web presence, by rapidly increasing web traffic, gaining more links and establishing your reputation as an expert in your field.