Suddenly I feel proud to be a British Citizen… are you ready? Good:
HMRC (the lovely people who take our tax off us) provide a fully documented API, see here, for filling in just about every form they have. Not only that, but they define a whole set of schemas and everything available here: http://www.cabinetoffice.gov.uk/govtalk/schemasstandards.aspx
All I can say is my estimation of the UK gov’t went up when I disccovered that.
Unfortunately that’s as far as it goes, but still, an API for interacting with the HMRC is very, very useful.
We also have data.gov.uk where an entire archive of government data is made available; I believe there are APIs for accessing it too.
Edit: I have just stumbled across another UK data source/API:
Ordnance Survey Mapping Data (openspace). Some things have already been done with openspace I believe; still, free access to OS’s maps to manipulate how you will is very, very good. Ordnance Survey maps of the UK are very high quality and come in varying levels of accuracy and for different purposes (for those who haven’t lived/travelled in the UK).
Edit 2: Look at this! The World Government Data Store API from the guardian.
A newspaper, writing an API…! I’m in shock. Sticking to UK themed APIs, here’s the API directory of the UK from programmableweb.