In a customer project, I needed to do lookups against the Norwegian postal register. Normally, I have made a table in a SQL database or had an XML file that I have query against. This has created several problems, like to keep the register updated.

GeoNames is geographical database available under a Creative Common attribution license. This data is accessible free of charge through a number of web services. One of the services you can query against is postal register for over 40+ countries. Send postal code and country code (ISO-3166), and get data like city, county, county number and postal code back.

I’ve created an interface to the webservice "PostalCodeLookup". You find the code here : http://snipplr.com/view/16844/postal-code/