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.
However, Ordnance Survey(OS), has released for free the entire postcode database for Great Britain (this excludes Northern Ireland and the Channel Islands) together with the Eastings and Northings for each code. From this information it is possible to get the latitude and longitude of a postcode and then using Google mapping find the address of the postcode.
Just enter a postcode in the textbox provided and the latitude and longtitude of the postcode will be copied to your clipboard. Paste this into the Google textbox and click the blue button and the street and town will be revealed to you.
You will need to download the OS database to your computer unzip it and copy the CSV folder to the program folder. The path is C:\Program Files\postcode2address.Download postcode2address
The OS database is of course free, but the postcode2address software is £9.99. The download above is fully functional but will cease to work after 30 uses.