IA and
IB are the two main players in town and have their own spaces and workshop programs.
The Tribe is also a major source of opportunity.
Along with IB's three primary ensembles and showcase shows they also have a number of house teams that perform there often. In general I find IB to be very open with their space - especially on Wednesday and Sunday nights.
I've only seen one IA show but have met many great improvisors that have learned / played there. I believe they tend to steer towards half-shortform / half-sketch shows but I could be mistaken.
I am part of The Tribe. They don't currently have their own space, but they do have two weekly shows (they've had weekly shows for over a year straight) and are looking to begin a workshop program. Currently The Tribe is composed of 3 longform ensembles, a shortform ensemble, a sketch ensemble, and a variety of showcase shows, side projects, and temporary shows. I moved to Mass in January and joined The Tribe in March - has been a good time.
Along with these there are a number of unaffiliated troupes and college troupes.
The real heart of Boston Improv is the building in MIT where we all hold our rehearsals at no cost to us. Any given weeknight there are at least three improv troupes there.
It is not NYC, but I think there are plenty of opportunities to perform whatever form of improv is to your liking.