ErnestMarples.com serves up free postcode data (just don’t tell Royal Mail)
http://uk.techcrunch.com/2009/08/29/ernestmarplescom-serves-up-free-postcode-data-just-dont-tell-royal-mail/ErnestMarples is basically a postcode database with an API sitting on top. It allows developers to build cool projects without having to pay Royal Mail (or Her Majesty) an arm and a leg for the privilege of the data.