Given a latitude and longitude, we can tell you your postcode. This is based upon the mySociety augmented version of the Code-Point Open dataset released as OS OpenData.
If you provide a Geohash of your location, we can return the postcode. For example, this is where I live: http://whatismypostcode.appspot.com/gcpv5d33u20f.
If your browser has a geolocation feature, such as Firefox or a mobile browser on the iPhone or Android, then we can tell you the postcode of your current location: try it out.
The real reason I built this website was to automatically fill in my location in the Transport for London journey planner.
This service is based on data that contains Ordnance Survey data © Crown copyright and database right 2010 and contains Royal Mail data © Royal Mail copyright and database right 2010.
It was developed by Léon Brocard.