I've been a big fan of Backpack since the day it launched, and I'm currently managing 8 or 9 personal projects with it as I type.
When 37Signals announced yet another cool web app for creating collaborative online documents, I was excited! Then, I started reading about Writeboard, and I lost some of my enthusiasm. Couldn't you just do this in Backpack?
Then I read more, and discovered it's core capability: Version control. It's basic (no merging, for example), but it'll do. And it also integrates with a BackPack account. I'm excited again!
I've been wanting to write about web-based writing tools for some time, but every week someone comes out with a new one (see Writely). Now 37Signals has jumped into the fray, and they are updating the product continually.
Give it a try! You can create a free account with two current writeboards. Backpack has a free option, too.