I'm copying and pasting a couple of posts fromm last year... the descriptions are not my own but are fairly accurate.
Bars near BU (roughly closest to farthest from school): The Dugout (our School Bar), Audobon (sexy city style), P.J. Kilroy's (tatoos and broken bottles of Pabst), An Tua Nua (your standard Irish hip-hop venue), O'Leary's (elderly people enjoying folk music), the BU Pub (not a summer place), Cornwall's (English pub in an Irish towne), Boston Beer Works (brewed on location), Cask and Flagan (go Sox!), Copperfield's (smells funny), T's Pub (big screen tv)
If you want to venture a little further into the downtown area towards Bolyston Street or Faniuel Hall... There's Tia's on the waterfront (more of a preppy financial district after-work hangout), The Living Room (also on the waterfront, nice bar with comfy couches), Pravda (a black pants lounge/bar/club, which is a little more European) or The Place (a rowdy but fun bar).
Finally, we held last year's Happy Hour at Great Scotts -- a come-as-you-are bar with $1 drafts every Wednesday. They have live bands most nights (I think) so it's best to get there early to avoid the cover charge and while it's still quiet enough to hold a conversation.
One other thing, the Fenway bars are great but probably should be avoided when the Sox are playing in town because they'll be very, very crowded.