Technology, Books and Other Neat Stuff

I like the ability for more than one person to be editing a document with realtime updates to the other editors/viewers ... is there anything similar in Microsoft's products (not really shared whiteboards, but more the ability for two people to edit, say, a design spec or (in my case) an overview page so that it contains the latest information from each of two or more authors)

At the moment we are either using wikis and merging, or sending spreadsheets back and forth or using something like Perforce and having to merge versions in order to see what other people have done to the document that I'm editing.