Instant (But Not Freeze-Dried) Karma
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The essence of the traditional New England clambake is that it is just a bunch of friends on (or very near) the shore making their own party for the evening. The fixings are very simple, and there are outfits that will cater a clambake at your home (relatively expensive) but I know of no commercial places where you can go to experience this traditional event. In a way, I like that, that it hasn't gone the way of the Hawai'i luau. But there are places where you can go and have lobster with the locals in very low key settings. These are not fancy, just checkered tablecloths on picnic benches with plastic utensils and paper plates. But the lobster is fresh and you are on the water. Two that you'll be near are Abbott's in Noank, CT near Mystic and Newick's in Dover, NH near I-95 and the NH/ME state line.
AZBuck
It's been a looooooong time!
Oh - this brings back memories! I haven't been to a clambake since I was around 5 years old, but I do remember it being a good time. We have family friends that have their own lobster boat, so the food was especially fresh.
There are some commercial places in Maine (saw something about it on Chronicle a few weeks back; this is a local program that many times has a segment called "Main Streets & Back Roads", and is a source of some good information), but generally you have to make a reservation, and for the most part, they run only up until Labor Day.
Here's a link to one of these places, just as an example that they do exist.