The Mixerlabs GeoAPI provides application developers with the ability to query the world.
GeoAPI services include a reverse geocoder; deep data about 16 million businesses and tens of thousands of points of interest; a writable layer for developers to annotate the world and do complex geo-queries; and location-enabled media layers (e.g., Twitter and Flickr). GeoAPI also recently added an iPhone SDK to speed up mobile development.
The GeoAPI makes it fast and easy for developers to generate a variety of location services (e.g. everything from annotation of tweets with nearby points of interest to FourSquare/GoWalla/Brightkite style check-in apps).