Kevin, one of the founders of Wufoo, does a great job of explaining how he grew the product by word of mouth. He built it from the trust of his users, most of which was from http://particletree.com/. It's fascinating how much emphasis he puts on interacting personally with the paid users and how they scaled that. Definitely a lot of hard work, but it paid off when they had some server trouble when Wufoo first launched - their old users trusted them and backed them up when some of the initial comments were negative.

Long story short, he didn't do any search advertising either.

Interview here: http://mixergy.com/wufoo-kevin-hale/