Pair Wiki-ing
Ever had to work collaboratively, concurrently on the same document in real-time? So far, the offering for dealing with collaborative documents authoring is rather oriented towards an asynchronous mode.
You have Microsoft Word which allows you to make revisions, changes, highlights. You can use the good old email system by sending mails in a ping-pong way (no concurrent modifications allowed). You may also use a versioning system such as CVS or Subversion to edit documents in parallel and merge both work copies.
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