This data comes from Boston's GIS Data Hub, a project of their Enterprise GIS department. As they say on their homepage, the city's MIS (Management & Information Services) Department plans to use this application as a foundation for additional maps and data sets over time.
The movement for cities to publish more data online is gaining steam, and the city of Boston is one of the major players. We look forward to working with Boston and other cities as they move to more open government. Contact us if you are a government official interested in sharing data.