Actinia is an open source REST API for geographical data that uses GRASS GIS.
Actinia Grass GIS API is a free Geocoding API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.
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Details from actinia.mundialis.de.

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Yes. The Actinia Grass GIS 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.
Yes. The Actinia Grass GIS API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Actinia Grass GIS API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Actinia Grass GIS API is served over HTTPS, so it's safe to use in production.