Time Machine: Leopard's best feature?
You can later browse the Time Machine copy of any folder by navigating a useful timeline. In this way, you can regress through previous versions of the folder. This is a fantastic way to keep from losing files or just do a simple form of revision control. If you need more time range in your Time Machine, it's only a matter of slapping on a new external drive.
Now, we realize this kind of backup and recovery has been done before, but when you see how sell it integrates with Finder and Spotlight, we think you'll be just as anxious as we are to get your hands on the new OS X.