Widbook is a platform designed to help people collaboratively create multimedia books. I reviewed the service last summer and since then it has received a couple updates of note. Widbook's editing platform now allows you to upload DOCX and TXT files. This means that you don't have to do all of your writing online. You could start your project offline and bring it into Widbook later. The second update to note is that Widbook now has a chat feature that you can use with your collaborators to discuss edits to your work while you're in a project. Of course, all Widbooks can still include pictures, text, and videos.
Widbook - Write, read and share! from Widbook on Vimeo.
Applications for Education
I love Google Documents for collaborative writing, but sometimes I feel like want more formatting options. If your students feel the same way then Widbook might be the collaborative writing tool for them try on their next writing assignments.