This basement bar place is actually in Cambridge near Harvard Square, not Boston. It's a Harvard stomping grounds, so expect a lot of Harvard students during the school year before sporting events. Very down-to-Earth, with non-uniform chairs and simple lamps on the tables. My one gripe is that they've got too many mirrors. It's disconcerting when someone on the other side of the restaurant behind you can tell you have a booger hanging out of your nose.
This is a basement bar, but they have an outdoors section too without any mirrors, only you can't drink out there. VT user Carmanah has a great description of Grendel's on her page. The title of this section comes from the fact that Grendel's uses monsters from Where the Wild Things Are as their logo. And remember, no smoking! Grendel's used to get pretty smoky...not any more with Boston's no smoking rules.
Favorite Dish Any of the half-price food during happy hour (5-7:30), M-F. The food is pretty good and comes from a lot of places. Grendel's also has an ok selection of beer.
During the school year, we stop here for beers occasionally. It's a casual place to hang out and when we've been there was mostly filled with graduate students.