"Marketing phenomenon that facilitates and encourages people to pass along a marketing message." (marketingterms.com)
Also known, as "Word-of-Mouth Marketing".
A combination between Viral marketing & Email marketing are "Tell a Friend" scripts.
I use FreeTellAFriend.com - it's free, easy to use and if you make an account with them, they offer also some interesting reports about the people who spread the word about your site, blog or forum:
I've used FreeTellaFriend with this blog and on my page with the free reports.
November 13, 2008 - November 13, 2009: The Big Brother of Internet Marketing (Make Money Online - Work From Home Business)! An unique exepriment! Watch me LIVE making one million ( dollars / euros / pounds ) in one year by working online!