Google Docs:
Create documents, spreadsheets and presentations online. Keep them private, or share them with others by simply inviting them by supplying an email.
The coolest thing we've seen lately? An app that incorporates your google docs (and sharepoint sites) into your OUTLOOK! Shout-out to Thomas Yung in Technology for this one!
Here is an example of a shared document appearing within Outlook. When changes are made to this document - all those who "share" it see the reflected, updated changes.