ExchangeRate-API.com offers an easy-to-use JSON currency conversion API with unlimited free requests. The service supports all 160 major currency codes and an incredibly accessible API request format. ExchangeRate-API has been a popular option for developers to get exchange rate data since 2010!
Fixer.io - Fixer provides a free, simple, and lightweight API for current and historical foreign exchange rates and currency conversion. The API tracks rates published daily by the European Central Bank.
exchangeratesapi.io - Foreign exchange rates API with currency conversion.
FXRatesAPI - Your Gateway to Real-Time Exchange Rates
Open Exchange Rates - Free, hourly-updated exchange rates - a simple, accurate, open-source JSON API. Free for personal use, a bargain for everybody else.
Freecurrencyapi.com - Freecurrencyapi.com is a free, reliable & feature-complete currency exchange rates API. 32 world currencies & daily update rates.
xChangeApi - Easy to use, free live updated foreign exchange rates API