If you are running OSX 10.5 and have Time Machine configured to backup your system and you see that you are missing an email that you may have accidently deleted, you can recover it very easily.

With Mail.app running and the front-most program, enter Time Machine and you'll see the starry background interface but instead of Finder folders to sort through, you are in Mail. You can scroll back to the date you remember having the email and then restore it. It will restore into a folder in the On My Mac category so you can re-file it where you want.

The same thing also happens when you are in Address Book and iPhoto. Time Machine lets you scroll through the Address Book and iPhoto interfaces finding the information you're missing.

Unfortunately this cool tie-in with Time Machine doesn't exist with all apps, but it can be really handy dealing with lost information instead of lost files.