Fixer
A foreign exchange rates and currency conversion API
...One of its key strengths is its simplicity, as it uses straightforward RESTful endpoints and JSON responses that can be easily integrated into web or mobile applications. Fixer also supports time-series and fluctuation data, enabling more advanced financial analysis and forecasting use cases. While a free tier is available with some limitations, more advanced features such as higher request limits and additional base currencies are offered through paid plans.