Any given place typically only has one brand of beer in growlers. For the benefit of Greater Somerville beer drinkers:
* Hillside Wine & Spirits on Boston Ave carries Berkshire Ale, IPA, and Pale Ale. They're all excellent, but I'm on my 3rd growler of the plain old ale in as many weeks and I still don't find myself craving anything else. (This is a high recommendation: I am pretty mercurial in my tastes. There's a nice rich caramel body to it.) Conveniently enough, they're also 5 minutes' walk away.
* CBC in Kendall typically has 8 taps and will sell you growlers of most of them. I work a block away, so I can just swap bottles on the way home. Of their regular offerings, I favor the porter and their pale ale, and at least one of their rotating taps is always worth getting.
* Kappy's at Wellington Circle carries Ipswich Ale from Mercury Brewing (perhaps better known for their Stone Cat line). From Kappy's I learned that you can't get the deposit back if you have washed off the sticky Ipswich label. (Other brewers print on their bottles, so it hasn't been a problem for me otherwise.) My petty irritation at this, the distance to Kappy's, and the fact that beer at least as good is easy to find mean I probably won't be going back to Kappy's for a while. Ipswich isn't half bad if you have the chance, though.