I recently stumbled upon a really terrific web-based tool for collaborative writing. It's called EtherPad. I find it to be particularly well suited for cases in which two or more people need to agree on a stretch of text or a small document. Chris and I just started using it for refining the snippets of text that appear throughout the FamZoo user interface. I can see where it would be fantastic for all kinds of writing situations - technical or otherwise.
I love simple, focused, useful tools like this. Kudos to the folks at EtherPad.
Check it out.