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. Also provides Group Lookups.
Updated on Sep 30 2016
Notes: Now requires OS X 10.9 or later running on a 64-bit Intel processor
Although Finder indicates that this is version 2.0, the developer states that this is version 2.5.