CurrencyBeacon
Real-time and historical currency exchange rates with free tier

Provides current and historical currency exchange rates with free plan 1K requests/month.
CurrencyFreaks 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.
Currency API for real-time exchange rates, conversion and historical forex data across 1000+ currencies. Free plan, 1,000 calls/month, no credit card.
Details from currencyfreaks.com.

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Real-time and historical currency exchange rates with free tier
Real-time and historical currency rates JSON API
The CurrencyFreaks API is provided by CurrencyFreaks (currencyfreaks.com). Currency API for real-time exchange rates, conversion and historical forex data across 1000+ currencies. Free plan, 1,000 calls/month, no credit card.
Yes. The CurrencyFreaks 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 CurrencyFreaks API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The CurrencyFreaks API supports CORS, so front-end JavaScript can call it directly.
Yes. The CurrencyFreaks API is served over HTTPS, so it's safe to use in production.