From the OpenStreetMap website: "OpenStreetMap is a free editable map of the whole world. It is made by people like you. OpenStreetMap allows you to view, edit and use geographical data in a collaborative way from anywhere on Earth." In Boston, it includes roads, buildings, and more: you can see the map for an example.
If you're interested in learning more about OpenStreetMap, and getting involved in helping to keep a free map of the world available, starting with your neighborhood, join us at the Bloc 11 Cafe in Somerville on Feb 14th and 15th. More information is available on the OpenStreetMap Boston wiki page.