Craving a big steamed lobster and lots of little steamed clams, and having my girlfriend's birthday as an excuse to cook extravagantly, I checked out Arlington Lobster on Mass Ave in (duh) Arlington, a couple blocks Cambridge-ward of the Capitol Theater. They're fairly new in the neighborhood - they can't've been there more than a month or so.
I was very happy with the experience.
Their lobster tank is a big, open-air thing, with lots of noisy circulation and crystal-clear water - unlike the scary science-experiment lobster aquaria that even a lot of upscale supermarkets and fish shops have. The lobsters therein were very much alive, peppy and alert - had they not been securely rubber-banded, I would have been nervous about them seizing control of the car on the way home, and driving us all back into the sea.
I got two 1-1/2 pound beasts, plus 1-1/2 lb of steamers, and steamed them (separately, sequentially, in wine, garlic, and fresh thyme) - they were as fresh and sweet and tasty as anything I've been served in a restaurant (plus the whole illicit thrill of the kitchen murder thrown in for free).
I've had bad luck in the past with lobsters bought further than easy walking distance from the ocean - they too often have this slightly-off chemically iodinish taste that you don't want to think too much about while you're eating them. If you live within easy walking distance of the ocean, you probably already have a good reliable source of lobsters. If you don't, but you live within easy driving distance of Arlington, I recommend Arlington Lobster.