OpenStreetMap

OpenStreetMap is a collaborative world map built by volunteers, with data free to use, modify and redistribute under an open licence.

That licensing is the point: commercial mapping data is expensive and restricted, while OSM data can be embedded in a product, processed offline or used to build a custom map without per-request fees.

Coverage quality varies by region — dense in Europe and thinner in places with fewer contributors — and it is the underlying data source for a large number of other mapping products.