Here's a very handy plugin for Address Book. PostCheck does two things. First, it fills in missing Zip Codes for when you've only got a contact's city and state. It's super simple. Just right-click on the empty Zip Code field and select PostCheck from the contextual menu. In my testing, it accurately provided the accurate 9-digit code across several cities and states. It also works when you've got the Zip Code but need the city and state.
PostCheck's other trick is to format your addresses so that they meet the US Postal Service's exacting standards. As everyone knows, it's not a good idea to tick off the mail carriers. It's called "going postal" for a reason, folks.
PostCheck requires Mac OS 10.4 or higher, is universal and costs $10US. It's definitely a useful tool to have around.