An extranet to be precise, but one that specialises in document distribution(/colloboration).
Yeah, I thought about that, but it would require a fair amount of management and discipline (much as our current local filing system does). I'm looking for something a little more self-managing.
Came across this, which looks interesting, but it's not open source and it's probably overkill (we already have a calendar/address book/filing system etc.), although the timesheets part would be useful:
http://www.copperproject.com/
Ideally Apple would build something into Mac OS X Server. That would probably be perfect.