Collaborative Writing Tools And Technology: A Mini-Guide

Google Docs seems to have the most bang for the buck ($0) 

The key features that characterize collaborative writing tools and technologies include:

• File types supported:which are the supported file formats for the files that can be edited collaboratively
• Text chat: the presence of a text chat that users can utilize to communicate while editing
• Revisions:the capability to track all changes made to the original document and go back to older versions
• RSS: support for RSS feeds, allowing users to get real-time notifications when changes are made.
• Email updates:the capability to receive email updating users when there are changes to the documents that are being edited.
• Public/Private:the possibility to set up private or public collaborative editing sessions.
• Max Editors:the max number of editors allowed
• Real-Time - Co-editing:the possibility to collaboratively edit in real-time
• Software/Web-based: the type of collaborative system
• Comments:the possibility to add comments to the document
• Spell check:the availability of a spell checking tool

Source: Collaborative Writing Tools And Technology: A Mini-Guide