The Sunset Cantina is a great place. A lot of people in the area head there after work for happy hour, where they have relatively inexpensive mixed drinks and cocktails, and a pretty good selection of beers on tap (though its sister restaurant, the Sunset Grill down the street, is the one known for having the biggest selection of beers on tap in Boston).
They also have excellent food, if you ever get a chance to try the restaurant part. They have the same menu as the Sunset Grill in fact, with a wide variety of choices, including some classics with a slightly "Mexican" twist like an added spicy salsa or something along those lines. The interior is a little dark for my taste, but you can also sit outside for great people watching on Comm Ave. Service is usually pretty good and quick, though when things get busier it occasionally slips a tiny bit.