Vinu Murugesan just launched a Collective Prose. A pretty interesting application for experimenting with collaborative writing:
Collective Prose is a place “where you can share your thoughts, learn new things, tell stories, and share insight.” Anyone can share their thoughts by writing posts. Collective Prose uses tags to categorize these posts which makes finding related content easy. Also, people can save posts they find interesting so that they may get back to them later
Murugesan did Collective Prose with Ruby on Rails and tells his story in Building Web applications intelligently.