Socratic Blogging. Two or more (real or fake) people have a conversation about a topic and it's posted as a blog entry. I think it'd make for a really interesting new genre of blogs very different from the traditional essay style. A web-based Ajax chat style writing system and an IM client plugin as well, would make it super convenient and painless to create new posts. Traditional bloggers might occasionally do these coop posts together, others might do this type alone.

I had this idea because friends and I sometimes talk on IM about a topic and the chat log turns out to make quite interesting reading.