OpenStreetMap is, of course, the obvious answer. There are various APIs for managing the data in the database, and there are various APIs for getting maps onto your webpages, such as OpenLayers, Mapstraction, or Staticmaps.
OpenStreetMap is, of course, the obvious answer. There are various APIs for managing the data in the database, and there are various APIs for getting maps onto your webpages, such as OpenLayers, Mapstraction, or Staticmaps.