Viral marketing, buzz marketing, stealth marketing -- the names vary, but its popularity is undeniable. JupiterResearch polled 273 U.S. advertisers in April 2006 and found that one in five of those that advertise online will try viral marketing in the coming year. The examples and advice here can help you join them.

But first, what is it? At its core, viral marketing is information customers want coupled with a friction-free way to share it. Because it's inexpensive and leverages a company's unique expertise, it's a good fit for many small and medium-sized businesses.