interactivity plugin? how to connect to other blogs in my network?

I’ve got my multisite all setup and running with sub-domains. One question, and this is the main feature I’m looking for: Is there is a plugin to allow interactivity between blog sites? I want to create a community like Livejournal where members can browse all blogs on my network and see content from others (such as new posts and comments) in a generalized way.

Thanks!