I'm working on a side project that needed a currency conversion API, and I'd originally started using https://exchangeratesapi.io/ which until a few days ago was a free API - https://web.archive.org/web/20210328014153/https://exchangeratesapi.io/
As of today, it is now a paid API, and says it's operated by apilayer (which was recently acquired by a private equity firm).
It seems like they've acquired a decent number of currency conversion APIs:
1. https://openrates.io/ 2. https://currencystack.io/ 3. https://fixer.io
And probably a few more.
It's almost amazing how private equity strategy works :)