PostCheck is a plugin for Apple's Address Book that provides an efficient, one-click way to lookup missing ZIP+4 Codes, Cities and States and will format addresses to meet USPS standards.
F451 I was hesitant at first about purchasing a license, but in my correspondence with the developer I made the purchase without reservations. Those who know the USPS system also know that this little software jewel can literally save days in sending mail via USPS. (Version 1.6)