A rather popular chain of burrito restaurants in the Boston area. IMHO, probably the best burrito in Boston.

They have several locations, one in Porter Square, one in Davis Square, and one along Beacon Street in Brookline.

Anna's is best known for its cheap, good, and filling burritos. They are available with a variety of toppings, ranging from steak to chicken to carnitas to simple beans and rice. Burritos are available in normal (10" tortilla size) and "super" (a larger 12" tortilla) sizes. A super size will usually be able to conquer even the largest appetite, and will run you at most a modest $3.75. And even better, the expert chefs will usually hand-make your burrito in well under 30 seconds.

Anna's also has tacos and "Mexican plates," but I've always stuck to the burritos.

Update: I've recently been increasingly attracted to the quesadillas, which are tastier than the burritos, I think. This is mostly on account of the fact that they are fried in fat and then more cheese is added. However, they take longer to make, fall apart more easily, and are in general a messier affair so far as eating them goes. But burrito or quesadilla, you can't really loose at Anna's.

The tradeoff for the dirt-cheap prices and delicious food is a slight deficiency in the decor and atmosphere departments. A few Mexican ornaments comprise the few bits of decor around, and the spartan tables and chairs can give almost a cafeteria setting. Not a good place for a romantic date, but a good place to go with a few friends and spend less than $5 for a great meal. Lines can be long around lunchtime on weekdays, but they're usually not too bad, as the extremely efficient assembly-line finishes most orders in well under a minute.

Details for the Porter Square location, which is the one I go to most:
822 Somerville Ave in Cambridge, Massachusetts.
(617) 661-8500

By the T:
Exit the Porter Square T stop on the Red Line via the main entrance, and orient yourself so that CVS is directly in front of you. It'll be out the door on the right leaving the escalator. Look to the right, where there's a building with a Pizzeria UNO, a Blockbuster Video, and a few other shops. Anna's Taqueria's yellow sign should be visible from here, on the opposite side of Somerville Ave.