Frankfurter

Frankfurter is an open source API for current and historical foreign exchange rates, published by the European Central Bank. Get the latest rates, the rates on any date back to 1999, or a time series between two dates, for about 30 major currencies, and convert amounts by choosing a base currency. Rates are updated every working day around 16:00 CET. Responses are JSON. Free, no key needed and no usage limits.