Alive & Kicking on Putnam St., Cambridge
I always liked their lobster sandwiches. Not lobster rolls, with those thick large hunks of lobster, but rather, lobster sandwiches. Lobster and (what I assume is) mayo, made into a thick chunky spread, between two slices of thick buttered and twice-toasted bread. It always fixed my lobster roll cravings, especially as it was around the corner from where I lived in Cambridge. $10 (not sure if it's gone up since) for the sandwich and a bag of chips. The guys who work there are really friendly. I don't remember the price of their seafood, but do recall picking up salmon there for far less than it was at Whole Foods. They sometimes had nice large plump scallops for sale too.
Picture of the lobster sandwich: