If your app or website needs to connect locations with administrative areas, like constituencies or counties, MapIt can save you a lot of time.
MapIt is mature, trusted and widely deployed. And it is available as a hosted API or a self-deployed web service, to suit your needs.
So 48.85, 2.35 becomes Paris.
Whether you self-host or use a hosted version, you get the data RESTfully.
Deploy an instance and fill it with whatever boundaries you want.
Versions pre-loaded with data exist for various areas.
The original MapIt, with UK data from Ordnance Survey, ONS, and more.
An expansion of MapIt for every point on the globe, using OpenStreetMap data.
With county boundaries from OpenStreetMap, and postcode data sourced by volunteers.
With data from the Australian Bureau of Statistics.