This tool uses OpenStreetMap data for the buildings and base layer.

Unfortunately, it kind of hides that it is doing that, which sucks. Usually the required attribution is placed in a corner of the map. Shade Map has a little information-i there which does appear to list OpenStreetMap according to my browser's dev-tools, but it seems to be broken and gets hidden behind other page elements, so only '© Shade Map |' is visible.