bng2latlong
Convert British OSGB36 easting and northing (British National Grid) to WGS84 latitude and longitude

Create maps and markers for anything.
Cartes.io API is a free Geocoding API that requires no API key, is served over HTTPS and has unconfirmed CORS support.
Create live, community-driven, and anonymous maps and markers for anything.
Details from github.com.

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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.
Convert British OSGB36 easting and northing (British National Grid) to WGS84 latitude and longitude
Brazil RESTful API to find information about streets, zip codes, neighborhoods, cities and states
Provides fast and accurate IP geolocation APIs along with security checks and confidence area
The Cartes.io API is provided by M-Media-Group (github.com). Create live, community-driven, and anonymous maps and markers for anything.
Yes. The Cartes.io 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 Cartes.io API requires no authentication, so you can start making requests without signing up.
CORS support for the Cartes.io API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Cartes.io API is served over HTTPS, so it's safe to use in production.