Being able to share, track revisions and collaborate on files is a must for any project; Google Docs is a free online service where you can securely create, store and share documents, spreadsheets and presentations and online forms. You can work on your own or collaboratively, with the ability to track and maintain revisions. You can also import docs from MS Office and Open Office.
The below video provides an excellent overview of Google Docs:
Some useful related links:
An online tour of Google Docs features.
Getting Started Guide for Google Docs.
Official Google Docs blog.