A collection of currency exchange rates (data in XML and JSON).
National Bank of Poland API is a free Currency Exchange API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
NBP Web API allows HTTP clients to make enquiries on datasets published by the NBP.PL
Details from api.nbp.pl.

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The National Bank of Poland API is provided by NBP Web API (api.nbp.pl). NBP Web API allows HTTP clients to make enquiries on datasets published by the NBP.PL
Yes. The National Bank of Poland API is listed in the Currency Exchange category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The National Bank of Poland API requires no authentication, so you can start making requests without signing up.
Yes. The National Bank of Poland API supports CORS, so front-end JavaScript can call it directly.
Yes. The National Bank of Poland API is served over HTTPS, so it's safe to use in production.