Time Machine is probably the defining feature in Mac OS X Leopard. It provides a nice, clean interface for you to backup and restore your files; but did you know you can also restore your computer from the Time Machine backup?
When you insert the Leopard install disk and boot off of it you will be presented with a semi-Mac OS X desktop. In the menu bar, select Utilities and then "Restore System from Backup..." Select your backup drive, the date you want to backup from, and then click restore.
James Duncan Davidson has a full guide on his website describing how the process went. He mentions that while it restores all the files, the caches and databases are not restored. This means when you launch programs such as Mail the application will need to recreate the database, which may take some time.