This is when you place an advert somewhere and let people spread the advert themselves: examples are placing a film on internet and other people linking to it, or sending emails with the link in it, or having people attractive to your target audience meeting the audience in bars and after a conversation innocuously talking up the brand you’re promoting, in the hope that that person will not only buy into the brand, but also tell others about the new brand. Red Bull was particularly succesful this way. William Gibson explains the concept well in his latest novel Pattern Recognition.
I found this due to the attempt to ban the Mortal Kombat: Shaolin Monks campaign due to excessive glorification of violence.
Another one that caught my eye especially was the Training films 1 – 3 But there are plenty more there for your enjoyment!