Apple’s Address Book is a central repository for your contact information and makes your contact information available to many other Mac programs. To enhance Address Book, take a look at an application called PostCheck, which adds some great improvements.
For starters, have an incomplete address in your contacts? PostCheck will find the ZIP code if you have city and state. And vice versa, if you have a ZIP with no city and State, PostCheck will find it for you. Additionally, PostCheck will take your address and conform them to the USPS’ exacting standards for addresses.
The program is a mere $10 and is available on PostCheck’s homepage.