The Open Exchange Rates API currently provides real-time, hourly-updated exchange rates and historical data for 161 world currencies that can be used in web and mobile applications, frameworks and with any programming language.
In addition to providing real-time currency exchange rates, the Open Exchange Rates API provides historical exchange rates data dating back to 1999. API responses are returned in JSON (JSONP support is also available) and all API calls must include an App ID which is provided after signing up for an account.
Per the official website, some of the features the Open Exchange Rates API offers include:
Applications currently using the Open Exchange Rates API are showcased on the API site and include WooCommerce Currency Converter, Calculator², iGotCharts + Units & Currency Converter, and Currency Exchange Fee Calculator.