Nominatim at https://nominatim.openstreetmap.org turns addresses and place names into coordinates, and coordinates back into addresses, using OpenStreetMap data.
None, but the public server's usage policy allows at most one request a second, requires an identifying User-Agent, and forbids heavy use — run your own instance for bulk geocoding.
Full reference: https://nominatim.org/release-docs/latest/api/Overview/
curl "https://nominatim.openstreetmap.org/search?q=Eiffel+Tower&format=json&limit=1"[
{
"place_id": 97665530,
"licence": "Data © OpenStreetMap contributors, ODbL 1.0. http://osm.org/copyright",
"osm_type": "way",
"osm_id": 5013364,
"lat": "48.8582599",
"lon": "2.2945006",
"class": "man_made",
"type": "tower",
"place_rank": 30,
"importance": 0.6205937724353116,
"addresstype": "man_made",
"name": "Tour Eiffel",
"display_name": "Tour Eiffel, 5, Avenue Anatole France, Quartier du Gros-Caillou, Paris 7e Arrondissement, Paris, Île-de-France, France métropolitaine, 75007, France",
"boundingbox": [
"48.8574753",
"48.8590453"
]
}
]Coordinates to an address.
curl "https://nominatim.openstreetmap.org/reverse?lat=40.7484&lon=-73.9857&format=json"{
"place_id": 358431881,
"licence": "Data © OpenStreetMap contributors, ODbL 1.0. http://osm.org/copyright",
"osm_type": "node",
"osm_id": 13306330095,
"lat": "40.7484257",
"lon": "-73.9856700",
"class": "tourism",
"type": "viewpoint",
"place_rank": 30,
"importance": 0.00009175936522464359,
"addresstype": "tourism",
"name": "",
"display_name": "350, 5th Avenue, Koreatown, Manhattan Community Board 5, Manhattan, New York County, New York, 10118, United States",
"address": {
"house_number": "350",
"road": "5th Avenue",
"neighbourhood": "Koreatown",
"suburb": "Manhattan",
"city_district": "New York County",
"city": "New York",
"state": "New York",
"ISO3166-2-lvl4": "US-NY",
"postcode": "10118",
"country": "United States",
"country_code": "us"
},
"boundingbox": [
"40.7483757",
"40.7484757"
]
}