A backup of Address Book!
There was a tip a while back about how to use iTunes built-in backup feature instead of a separate application such as SuperDuper or Backup. iTunes wasn't too bad anyway, because you can just backup the MP3s and AAC files. An application such as Address Book is entirely different. You cannot save or even really copy files. There is also a built-in backup option in Address Book. What I really like about it is that you can restore from your backup afterwards and you don't have to reorganize anything. It is back to the original state! You can find the backup feature in Address Book under File and "Back up Address Book...". You can then select where you want to save the backup. What is great about that is that unlike iTunes, the file size will remain small and you can back it up in multiples locations (preferably at least one on a remote server (the web)).