These free online office suites are among the most fully developed Web applications out there, featuring full-fledged word processors, presentation editors and spreadsheets that support Microsoft Office formats. (The Acrobat.com Tables app combines some functions of both a spreadsheet and a database.)
Each service makes it easy to invite others to participate and grant them permission to edit, comment on and/or review the documents you've shared with them.
All three of these services allow real-time collaboration in word processing documents and spreadsheets so that multiple people can edit a file simultaneously. In Google's services, color-coded cursors or spreadsheet cells indicate who is working on what. Zoho locks the paragraph or cell each person is on so that changes don't overlap. Google Docs and Zoho both offer an integrated chat "sidebar" so you can converse with your collaborators while you work.
Acrobat.com has the weakest real-time capabilities. In Buzzword, the word processor, the whole document is locked until the editing collaborator saves his or her changes, while Tables allows multiple people to work in the same cell but keeps only the changes made by the person with the highest permissions level (the document's "author"). Further, users cannot chat in the interface where they're editing, though a ConnectNow conference in a separate tab might be adequate.
None of these suites allows real-time editing of presentations, though multiple people can view a presentation at the same time while one person edits it.
Zoho offers a wide variety of productivity and business tools beyond the Big Three office apps, as does Google under its Google Apps service.
It's worth noting that Microsoft will be releasing its own online office suite, called Office Web Apps, this year, but it's too soon to tell how polished its collaboration features will be.
TextFlow from Nordic River Software simplifies the process of integrating changes into Word documents by combining several versions of the same document into one and highlighting the differences so you can easily choose among them.