There's nothing really fun about an address book. It is after all, well, just an address book -- how exciting could that possibly be? Better yet, how do you make it more useful than it already is?
A small plug-in from emlyn.net may do the trick if you use Apple's AddressBook and have it full of phone numbers without addresses. Installation of the plug-in is easy -- just drag it to your Library > Address Book Plug-Ins and restart AddressBook.
By Control-clicking on a phone number and choosing Reverse Look-up Plug-in you can fill in the address information automatically.