Plotbot

Plotbot is a free on-line collaborative screenwriting tool:

As film school graduates who both work on the web, we thought it would be a lot of fun to have a site where people could write a screenplay together. At first, this was going to be a small, simple application. And then it got out of hand.

We do think that we're on the way toward making a great collaborative screenwriting tool. The public projects are a lot of fun, and the private projects allow people to write together more productively than traditional screenwriting tools.

I don't write with other people, so I haven't tried it yet, but I can see where it could be very useful.