Nominatim (OpenStreetMap)

Nominatim is the search engine of OpenStreetMap. It turns an address or place name into coordinates (geocoding) and coordinates back into an address (reverse geocoding), worldwide, with results in JSON, GeoJSON or XML. The public server is free to use but has a strict usage policy: at most one request per second, a User-Agent or Referer that identifies your app, no bulk geocoding, and attribution to OpenStreetMap. For heavier use you can run Nominatim on your own server.