Etherpad is an on-line collaborative text editor in which up to 8 users can simultaneously participate in producing documents in real time. It was devised by a couple of ex-Google employees and is wonderful in its simplicity.

I've tried it and provided your internet connection is fast enough it works like a charm. You can save the documents on-line and then cut and paste them for use in whatever word processor you happen to favour. It doesn't handle images or other objects yet, but I suspect that's only a matter of time.
For those of you who might need a more 'whiteboard' type of approach which can handle drawing you might like to try Twiddla or you can check Etherpad out here
Anyway, I'm off to celebrate Rene Magritte's birthday with one or two of these: