Whatever else you might normally write  

This new Writeboard service (via this Monkeyfilter post on NaNoWriMo) has a lot of potential for the kind of collaborative creative applications that I'm interested in, but also for business productivity. Basically it's a scaled down, simplified wiki that's password protected and document-focused (as opposed to community-focused). It's the closest thing I've seen to an online word processor, with the infinite version history standing in for both the undo and track changes features. I think if they added in some basic but elegant formatting capabilities and a convert to PDF function, this would really be a killer app for the workplace.

As far as collaborative fiction or poetry, I think it might work well at the level of a few collaborators, where groups can enforce their own rules and procedures for how final decisions are reached. For large scale collaborations, though, I think some kind of administrative system would be required -- probably something like voting, though possibly built into the background via measures of user behavior (clicking, linking, and the like).