MapIt
Look up administrative boundaries
MapIt is mySociety's web service for looking up the names and locations of administrative areas.
It's useful for anyone who needs to turn a longitude/latitude or a postcode into the name of a council, constituency, region or state. MapIt is a powerful, practical piece of web infrastructure, widely used across a variety of websites.
Originally built just for UK users, MapIt is soon to launch as a global platform, to allow administrative boundary look-ups anywhere in the world.