It’s the first of April – but it’s not an April Fool – lots of Ordnance Survey medium-scale data has been released today, under a licence compatible with Creative Commons’ Attribution, i.e. you can do what you like with it as long as you attribute and don’t misrepresent the data source.
The best mirror for the data I’ve found is at MySociety – the OS’s own servers have been apparently overloaded since the release went live.
The first use I’ve made of the data is taking the “CodePoint Open” set of postcodes and locations, I’m now using this data as the postcode lookup for OpenOrienteeringMap. If you type in a UK postcode there, it should now take you to exactly the right place. Before, the lookup was using NPEmap data, which was pretty good in general, but someone did spot some glaring errors when they were using it, coincidently, yesterday.
The attribution statement, by the way, can be seen by mousing over the “Jump to Postcode” text.