Xconomy.com maintains a list of law firms serving venture and startup communities. Check it out here.
http://www.xconomy.com/boston/resources/law-firms-serving-venture-and-startup-communities/
There is also a fairly stale post (dated August 24, 2011) by chubbybrain.com that lists the top 20 Boston startup lawyers at that time.
http://www.chubbybrain.com/blog/boston-startup-lawyers-top-20-massachusetts/