Broadest collection of public data.
Enigma Public API is a free Open Data API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
Details from developers.enigma.com.

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The Enigma Public API is provided by Enigma v1 (developers.enigma.com).
Yes. The Enigma Public API is listed in the Open Data 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 Enigma Public API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The Enigma Public API supports CORS, so front-end JavaScript can call it directly.
Yes. The Enigma Public API is served over HTTPS, so it's safe to use in production.