I'm with AnnaS. Unless you have a compelling reason to take your car into Boston, ride the T (subway).
It's laid out very well, it gets you to almost everything of interest in the city, and is pretty safe.
I live only 2 hours west and my mom still lives in Cambridge. Whenever I travel into Boston, I park the car at my mom's, walk down to Harvard Square and grab a train.
For someone not lucky enough to have a nearby relative, there is abundant parking at Alewife (Cambridge - Red Line) and Riverside (Newton, Green Line - right off Route 128/I95).
Another strategy is to take your car in town and park at the Boston Common underground garage. You can walk the Commons and Public Garden (don't forget the swan boats if you're with the kids or the significant other!), you can ride the train to virtually anywhere in town, and return for your car at the end of the day.
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"I will not be pushed, filed, stamped, indexed, briefed, debriefed or numbered. My life is my own."