Google Docs supports multiple simultaneous editors and tracks revision histories. It was a lifesaver for group projects in college.

And while 'always online' is often a bad thing, it seems to be a lot more reliable than the alternatives of email chains with the attached Word document, appending -R3-edited-complete-final-done.docx to the filename, the manual merges when two people are working on the same thing, or the USB stick that gets passed around.