Google-drive – How to edit existing .docx in Google Docs without losing columns

There's Etherpad, which is a GPL collaboration verion of Google Docs. There are various example implementations which protect the privacy of your documents, although privacy isn't the issue.

Almost three years later, Google Docs now natively supports the editing of DOCX files stored in Google Drive, though changes made to a Word document are not automatically saved. (That said, saving changes is a matter of clicking a button.)

If two people are working on the document at the same time, Docs will notify the other person if the document has been saved while they are working, but if the second person saves their changes later, the version saved last will overwrite the previous one.

Since Google Docs lets you know who is currently viewing the document besides you, if you notice another person working on the document, it would be of mutual benefit to communicate with your colleague—perhaps using the built-in chat system—to make sure that neither of you overwrites the other's hard work.