Boom Writer. Since then I've learned a little more about it and I am really quite pleased with it. Here's what Boom Writer is: Boom Writer provides a "starter chapter" for a story and students continue the story by writing additional chapters. The chapters submitted (anonymously) by students are voted on and the chapters receiving the most votes make it into a book that students, teachers, and parents can choose to have published by Boom Writer.
Here's a short video introduction to Boom Writer.
WGBH Boston ran a ten minute feature on Boom Writer in schools last year. You can watch that video below.