Place names and other geographical data.
GeoNames API is a free Geocoding API that requires no API key, is only available over HTTP and has unconfirmed CORS support.
Details from geonames.org.

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The GeoNames API is provided by GeoNames Web Service Documentation (geonames.org).
Yes. The GeoNames API is listed in the Geocoding 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 GeoNames API requires no authentication, so you can start making requests without signing up.
CORS support for the GeoNames API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
No. The GeoNames API is only available over plain HTTP, which browsers block on HTTPS sites. Call it from a server instead.