I use google docs for collaboration quite regularly both in school and outside of school. However, as a middle school teacher, one problem that I face with google docs for student use is that you need an email (preferably a gmail account, though not necessary) account to participate or even read or have access to the documents created. The collaborative aspect of google docs is great, but since you need to know the email of the person or persons with whom you'd like to share, this can become a problem for sharing with students. So, I searched a bit and found a bunch of similar collaborative forums where students might collaborate on a document through a web link.
One of these sites is called TitanPad, a basic web-based word processor that allows collaborators to see changes to a document in real-time. There are no email accounts needed, though it is possible to share via email. Once you have started a new "pad" you are given a web link to send out via email or post onto a webpage. Users will simply go to the website and read and even add to the document. A quick example is the one I started and can be viewed at http://titanpad.com/YvPiB0Y4T4. Your students can view and even add or change the document. Each new member will receive a different color to distinguish his/her changes.
This forum would be extremely useful for students who are writing together. They can start a new document or import a template or their document already in progress. They can access the document from home or from the classroom and then can export the document in order to save it as a file. As a teacher, I could use this in a science lab, for example to create a data table for students to add to as they collect data at lab stations. Each group could then see the data added to the document. And, I could create a separate link for each of my individual science classes.