Still on viral marketing, MarketingSherpa has an excellent list of their top 12 viral marketing campaigns here, covering a variety of businesses. Good for idea-sparking.
And I strongly recommend you sign up for MarketingSherpa’s free email newsletter while you’re there. It’s a mine of tips, case studies and other marketing goodness. They also produce some very thorough studies and run some excellent conferences — I attended their Email Marketing one in Chicago this year and found it invaluable.