Provides forward/reverse geocoding and batch geocoding.
LocationIQ API is a free Geocoding API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
LocationIQ documentation for Introduction.
Details from locationiq.org.

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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.
UK postcode lookup with lat/lon, district, ward, constituency
Get the IP geolocation data through the simple REST API. All the responses are JSON encoded
The LocationIQ API is provided by LocationIQ (locationiq.org). LocationIQ documentation for Introduction.
Yes. The LocationIQ 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 LocationIQ API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The LocationIQ API supports CORS, so front-end JavaScript can call it directly.
Yes. The LocationIQ API is served over HTTPS, so it's safe to use in production.