OpenStreetMap (OSM) is a collaborative project to create free editable geographic data of the world. Volunteers gather location data using GPS, local knowledge, and other free sources of information and upload it. The resulting free map can be viewed and downloaded from the OpenStreetMap site: www.OpenStreetMap.org.
The data from OSM can be used in various ways including production of paper maps and electronic maps, geocoding of address and place names, and route planning. The data is owned by OSM but they allow anyone to use them as intended. So here is a way you can visualize OSM Buildings data in unity with nothing but a simple REST API which is also available on OSM website.
This plugin solely focusses on fetching data from osmbuildings.org, interpret it and then render the result as 3D meshes. This plugin does not generate any data or claim the ownership of any data.