Some stats are available on Trulia: http://www.trulia.com/real_estate/Boston-Massachusetts/commu
Try this website http://www.zipskinny.com
Also Realtors have acces to reports that break down everything for you. If you wish just e-mail me with your the name of the place and I can PDF the info to you. But again the local PO is the best source.
Most city PD's have this info available online by the way.
If you're in Charlestown as your Trulia Name suggests, then you probably know that Boston's crime statistics can be somewhat misleading. Crime statistics that blanket an entire neighborhood like Roxbury 02119 might make all of Roxbury appear to be dangerous. But pockets of Roxbury like Fort Hill and Mission Hill are actually quite safe. The South End 02118 is one of the most desirable neighborhoods in town, but then there's also a number of homeless shelters, methadone clinics and public housing projects scattered around the South End. High numbers of auto break-ins on desolate Harrison Ave and high rates of assault in the housing projects can skew the statistics for the entire neighborhood.
The Key: Get a broker who truly knows the intricacies of the specific Boston neighborhood where you're interested in buying property. Boston brokers are not legally permitted to tell you if a neighborhood is considered safe, but a good broker knows how to talk around the issue.