TextFlow is a new parallel document editing application and service that allows multiple users to work on a document simultaneously. The service should appeal to people who use the Track Changes feature of Microsoft Word. TextFlow lets users edit documents, and instantly see the changes suggested. One person can review the changes and apply them.
The difference between this service and something like Google Docs or working in a wiki, is that the changes are indicated visually, rather than having to go through the diff displays in revision history. It's simple and should be intuitive for users who use Track Changes in Word.
The service is currently in beta, and you can view a screencast demonstration on the TextFlow site.