Economic data from the Federal Reserve Bank of St. Louis.
FRED API is a free Finance API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
Details from fred.stlouisfed.org.

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The FRED API is provided by St. Louis Fed Web Services: FRED® API (fred.stlouisfed.org).
Yes. The FRED API is listed in the Finance 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 FRED API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The FRED API supports CORS, so front-end JavaScript can call it directly.
Yes. The FRED API is served over HTTPS, so it's safe to use in production.