Cheap beer, respectable selection, live music, pool, and darts all in the basement of historic Memorial Hall...enough said.
Actually not quite enough; the bar is run by Harvard College so there's definitely a young feel to it (but if you're in to that kind of thing...). In all honesty it actually is a decent place to grab a beer but I'd pick a night there isn't a scheduled event or the place can get crowded. Fun fact: the pewter steins hanging on the back wall were sold when the pub opened but pretty much every one within an arm's length reach of the bar has been swiped. Challenge, anyone?
As far as I know, this is the only on-campus bar at Harvard. It's a bit hard to spot given its underground location but well worth the search. Apart from having a decent beer selection at the lowest prices in the area, Queen's Head usually has some sort of entertainment. For example, last Thursday had trivia night followed by a live band. (Not a great band, but the bar is big enough that you can largely avoid the music.) There's also a pool table and dart boards in the corner.
The bar also serves food, though I've never tried it. Queen's Head is usually fairly busy, though I've never seen it too crowded to find a seat. The only catch is that you need to be with someone who has a Harvard ID to get in. This place is for students.