Rachel, being a geeky writer, immediately saw the potential of using Subversion for writers. Her tutorial walks you through what Subversion is, how to install it on your Mac (if you're running Leopard it is already installed), some apps you can use to make interacting with Subversion easier, and how to track your changes. To be clear, this method is not for everyone. If you aren't comfortable with the Terminal you should steer clear of this, and if you don't revise the same document over and over again you'll have no need for it. That being said, it is a very clever idea.
[via the Hog Bay Software Forums]