Glad to hear differences of opinion on Martha's in Nashua. Years ago the place sucked, but they brew very good beers now, and I'd definitely recommend it if you're in the Nashua area. I wouldn't call it a destination brewery by any means, but the brewmaster does good work. Plus, M-F, 4PM-6PM, they have the best Happy Hour north of FL - buy one, get the second for a penny, on every drink but martini's and shots.
In Lowell, there's a bar called Ward 8 (http://wardeightlowell.com/) that has a good tap selection, and the food's not bad either.
There's also Lowell Beer Works brewery right off the canal in Lowell which isn't bad (part of the Boston Beer Works conglomerate).