ZipCarton is a plugin for your Mac OS X Address Book that tries to find the ZIP or postal code for the city you entered in your contact's details.
ZipCarton is completly free and can find postcodes in 45 countries: Andorra, Australia, Austria, Bangladesh, Belgium, Bulgaria, Brazil, Canada, Croatia, Czech Republic, Denmark, Faroe Islands, Finland, France, Germany, Greenland, Hungary, Iceland, India, Italy, Japan, Liechtenstein, Luxembourg, Macedonia, Mexico, Moldova, Monaco, Netherlands, New Zealand, Norway, Pakistan, Poland, Portugal, San Marino, Slovakia, Slovenia, South Africa, Spain, Sri Lanka, Sweden, Switzerland, Thailand, United Kingdom, United States and the Vatican (using Geonames).
Available in Danish, Dutch, English, French, German, Italian, Japanese, Portuguese and Spanish.