When I first heard about viral marketing, I thought it had something to do with spreading the word through a virus! Later on when I slogged my way through its index, I was surprised to find I was not far off the mark.
Although it has been around for quite sometime, people really began appreciating the worth of viral campaigns when they saw just how successful Google was with the GMail invite campaign.
The limited number of invites made you feel that it was a huge deal. I was moderating a forum when I received my invite from the forum owner and remember feeling on top of the world.
This is exactly the feeling viral marketing techniques aim to capture. With the proliferation of social networking and bookmaking, my bet is that viral campaigns will assume more importance than ever before. This is because eventually search engines are bound to become smarter and just ignore websites that have no useful content.
Mind you, viral marketing is NOT spamming. The first thing needed for a campaign is a list of people/websites you think are interested in you niche. The idea is to get these people hooked on what you are offering so they can "talk" to their friends and mates about your site.
There are people who swear on a certain technique, but then one man's meat is another man's poison, eh? So stick to the basics initially and then when you find that something works for you, stick with it.
Over at Digital Point forums, znitch-search has an interesting post "10 Contagious Viral Marketing Techniques"
Feel free to add to this post what has worked for you and more importantly what has not!