Google Wave is a forthcoming "collaboration and communication" tool from Google. From the preview screenshots, I'd say it looks a little like some other collaboration tools I've seen, especially tools like Blackboard that are used by academic insitutions. The "real time" aspect looks to be a newish wrinkle though.
Like most Google tools, it's extensible, with it's own API.
According to mashable.com, we should be hearing more about this tool soon.