Apigee - Intelligent and complete API platform
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.
Apiary - Collaborative design, instant API mock, generated documentation, integrated code samples, debugging and automated testing
Open Exchange Rates - Free, hourly-updated exchange rates - a simple, accurate, open-source JSON API. Free for personal use, a bargain for everybody else.
Postman - The Collaboration Platform for API Development
Currencyapi.com - Currencyapi.com is a free, reliable & feature complete currency exchange rates API. 170+ world currencies & 60 second update rate.