CurrencyFreaks
Provides current and historical currency exchange rates with free plan 1K requests/month
Real-time and historical currency rates JSON API.
CurrencyScoop API is a free Currency Exchange API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
Developer documentation for real-time and historical currency exchange rates on CurrencyBeacon
Details from currencyscoop.com.

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Provides current and historical currency exchange rates with free plan 1K requests/month
Real-time and historical currency exchange rates with free tier
The CurrencyScoop API is provided by CurrencyBeacon (currencyscoop.com). Developer documentation for real-time and historical currency exchange rates on CurrencyBeacon
Yes. The CurrencyScoop 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.
Yes. The CurrencyScoop API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The CurrencyScoop API supports CORS, so front-end JavaScript can call it directly.
Yes. The CurrencyScoop API is served over HTTPS, so it's safe to use in production.