There are several such examples. Enlisted below are some most popular ones:
Google Maps API : These are designed mainly for mobile and desktop use with the help of a flash interface and JavaScript.
Amazon Advertising API : Amazon is known for their products and thus their advertising API accesses their product to discover their functionality and thus advertise accordingly.
Twitter API : The API for twitter is usually in two categories, one for accessing data and the other for interacting with the twitter search.
YouTube : This API used for YouTube includes various functionalities including videos, live streaming, player, etc.
