The best - and cheapest - linguine and mussels we've had can be found at a small restaurant in Cambridge near Harvard Square. For a number of years, we would visit Il Panino every Saturday of the summer when both mussels and tomatoes are in season. It is on Saturdays that Il Panino has the linguine and mussels as a special for $5.95. The only place you'll find this special mentioned is on their paper menu, but the people behind the counter will know what you're talking about.
The dish comes with either a red or white sauce, but if you ask instead for "garlic and oil", you'll get our preferred version of the dish: al dente De Cecco linguine (which, along with Barilla, is one of only two dried pastas worth buying), a dozen or so mussels, garlic, diced plum tomatoes, a sprinkle of parsley and a dash of pepper, cooked with oil and a bit of butter.
Il Panino is part of a group of restaurants that includes the Trattoria Il Panino in the North End and Faneuil Hall, but the one on 1001 Mass Ave near Harvard Square is the only one where we've tried the linguine and mussels special. Even now, a year and a half after we've moved away, we make it a point to have our Saturday lunch there whenever we visit.