Collaborative Writing

Applications for Education
Students working in groups could use these creative tools to create outlines of lectures,share the burden of taking notes, collaboratively write a short story or create a subject-specific mind map.

Great collaborative site for MS Office like applications. 
Link:  Teach Collaborative Revision with Google Docs
step-by-step tutorial
revision features of Google Docs
* Registration with Google account required.

Simple collaborative documents with no registration.
Online collaborative mind mapping.  Awesome.
*Individual Registration Required.

Co+Create haiku, poems, short stories, flash fiction, novellas, exquisite corpse and songs, real time or asynchronously with your computer, tablet, cell phone, IM and even twitter.
*Individual Registration Required.  Can register with Facebook account