Cathy has been telling me about a collaborative novel at a site called scriblist.com.
Once the 5 winning chapters have been chosen, they will be published on Scriblist.com. The second submission window will then open and you will be able to write what you think should happen in chapter 2 in the story of your choice. You could of course try to write a second chapter for all 5 stories....This process will be repeated until we have 5 complete stories of approximately 6 chapters each."
Cathy had submitted her version of chapter 2, and although it was not selected, it was shortlisted. Someone from the site sent her a very nice email, providing feedback specific to the content of her chapter whch she found useful, and encouraging her to submit a chapter 3 (which she's been doing today). Impressive. See the scriblist site for advice for writing, details of timetables and how to submit.