Fixer.io is described as 'free, open-source foreign exchange rates and currency conversion JSON API. It returns current as well as historical data and is updated daily'. There are six alternatives to Fixer.io, not only websites but also apps for Self-Hosted solutions and SaaS. The best alternative is exchangeratesapi.io, which is both free and Open Source. Other great sites and apps similar to Fixer.io are xChangeApi (Freemium), ExchangeRate-API (Freemium), BankExchange (Freemium) and Open Exchange Rates (Freemium).
Foreign exchange rates API with currency conversion. Exchange rates API is a free service for current and historical foreign exchange rates published by the European Central Bank. Fully Fixer.io compatible with JSON & CORS support.