The Postcode Address File is a datafile of UK Postcodes and the appropriate street addresses, kept up to date by Royal Mail.
The datafile itself is licenced for a hefty sum - £3,750 for an unlimited user per click licence, £75,000 for a corporate licence and various prices in between for varying uses.
This licence fee only includes the data file, so you need to find a way to integrate it with your application yourself.
One of the problems with this is that non profit organisations, small businesses and individuals quite often cannot raise the funds to licence the PAF, or devote the resources to integrating it with their existing systems.
The Open Postcode DataBase aims to try and build a freely available database containing UK street addresses and postcodes, with an easy to use API to integrate into any application, desktop or web based. Since there are over 27 million possible addresses for the UK, this project needs the support of people like yourself, people who are interested in using the data or API, to submit postcodes and street addresses to the database.
We dont want house numbers, and dont collect any data that could be used to identify you from the postcode so please, if you are interested in using this data for free, submit your postcode to the database and help us build a complete index of the UK.