Podcasting
Hatip to Scobelizer on this one.
PodcastAlley.com -- the place to find podcasts. Ben Hammersley (of MovableType fame) and Tim Curry described this in February 2004 and by August 2004 it was a real product. Now it has millions of users (I'd guess) by November 2004.
How is that for viral. Like most great innovations, it is built on a new platform. That is, the availability of the iPod and iTunes as the 95% and 70% respectively leader in distributing audio.
What is it? Well, you take RSS (the real meat behind blogs) and combine it with the fast internet for downloads and suddenly, you can get all kinds of new content.
I am using DopplerRadio right now, but the leader seems to podcaster so I'll have to try that.