Wikipedia for Web APIs, OpenAPI/Swagger specs for public APIs.
APIs.guru API is a free Development API that requires no API key, is served over HTTPS and has unconfirmed CORS support.
Interactive API documentation for the APIs.guru REST API. Explore endpoints and test API calls.
Details from apis.guru.

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Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Yes. The APIs.guru API is listed in the Development 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 APIs.guru API requires no authentication, so you can start making requests without signing up.
CORS support for the APIs.guru API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The APIs.guru API is served over HTTPS, so it's safe to use in production.