The Bitfinex Go SDK by Bitfinex allows to access the API to integrate financial features associated with bitcoin and crypto-currencies. Official implementation, REST API, and WebSockets API available. It includes installation and usage.
Traindom, information product creation and marketing platform, offers many of its feature via the Traindom API. At the core of Traindom stands a belief that "anyone can start a business." Whether you want to create a membership site, provide a training class, or produce/market an information product of any type, Traindom looks to ease the time to market. Its API allows developers to access its platform within existing applications and tools. Accordingly, users are not locked in to utilizing Traindom for every last detail of their go to market plan.
The travel site Orbitz has partnered with Google to show a virtual walking tour of the neighborhood around a hotel. You and I know it as the Street View API, part of Google Maps (our Google Maps API profile).