US Census site contains Topologically Integrated Geographic Encoding and Referencing system (TIGER) street index files from original 2000 source to 2007. Prior years have been in proprietary US Census Tiger format - which can be converted to other formats with OGR2OGR among other tools.
The 2007 version released on March 31, 2008 is in shape format and also includes some structural changes and name changes to fields.
NEW! The 2010 version release is trickling in. A good chunk of the states are already in place. (NOTE: the 2009 dataset full). Massachusetts set can be found here and Boston specific layers.
In addition to streets and roads, TIGER contains landmarks, waterways, census tract divisions, zip + zip4 breakouts, county boundaries.
Link to Massachusetts Suffolk County files (includes Boston): http://www2.census.gov/cgi-bin/shapefiles/county-files?county=25025. These have Census Blocks and Block groups, landmarks, water,, Street index, and ward precincts (Census 2000 Voting District)
