I just completed a first draft of a document using Google Docs, the collaborative editing suite for the Web. I was editing with one other editor, and we were both making some quick changes, but Docs managed to keep up with 99% of them, only occasionally displaying a conflict notice. The system is pretty sturdy, and it tells you who’s editing with you, something MediaWiki won’t do (at least, not without an extension).
The formatting options aren’t so great, but the suite is still in development, so such things are to be expected. Right now it won’t replace Microsoft Word or OpenOffice.org, but it’s good for basic word-processing with collaboration from people all over the world. I had a document open in three states, and the system just kept on merging changes. This must be one descendant of Google’s JotSpot purchase. I’ll definitely keep this in my toolbox.