In previous postings I've talked about the processes that help take an idea viral, but we see successful ideas take off all the time without a structured process. These ideas are intrinsically viral. One example, my wife pointed out to me recently is the online fashion retailer ASOS (previously called As Seen On Screen). It has a simple idea: it helps you look like a celebrity without paying celebrity prices. You can buy a dress for £45 similar to one that Victoria Beckham obviously paid a lot more for.
It's working for them as the number of searches for ASOS on Google is showing a steady upward trajectory. ASOS do advertise, but the gradient of the chart shows something much more organic than the sudden spikes you'd get with advertising bursts. This feels like a word-of-mouth phenomena. Some of its traffic is carried through magazines (how my wife heard), but a lot feels like it comes from women passing on a good idea to friends and family (how my wife's sister heard).
Some ideas are intrinsically viral, or sticky as Malcolm Gladwell terms it, and we know it when we hear it. Other ideas need a little more help to take off.