Address Book.app - be sure to back up frequently!

Ahhh, the joy of computers. Address Book.app on Mac OS X just froze up while I was editing a contact. While it was "not responding," Mail.app, iCal.app, and Adium.app also refused to respond, and I had to force-quite them all. When I launched Address Book again, all the data was gone. Luckily, I back up often -- and you should, too. :)