The exact method of Viral Marketing implemented for a campaign will vary depending on your demographics, but some examples will certainly include:
Reviews of your Product
Blogs on subject
Competition with a 'real' prize and an incentive for entrants to pass on the message, such as being given additional entries in return for suggesting a friends' email address.
Opt-in Email Marketing containing:
Latest news or information
Cost comparison sites
The above is a list of implemented steps, what we are appealing to is certain human elements and this means certain marketing actions and the harnessing of independent online vehicles for spreading the 'word'.