
Met Museum of Art.
Metropolitan Museum of Art API is a free Art & Design API that requires no API key, is served over HTTPS and does not support CORS, so it should be called from a server.
Details from metmuseum.github.io.

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The Metropolitan Museum of Art API is provided by The Metropolitan Museum of Art Collection API (metmuseum.github.io).
Yes. The Metropolitan Museum of Art API is listed in the Art & Design 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 Metropolitan Museum of Art API requires no authentication, so you can start making requests without signing up.
Not directly. The Metropolitan Museum of Art API does not allow CORS, so call it from a server, serverless function or proxy instead of front-end JavaScript.
Yes. The Metropolitan Museum of Art API is served over HTTPS, so it's safe to use in production.