Hog Island has got to have one of the best happy hour deals in the city. $1 oysters, $3 beers, and an added bonus of outdoor seating with a great view of the bay. The lines can look daunting, but the wait isn't actually very long.
The oysters are super fresh and served up on ice with plenty of tabasco, lemon, and their sweet/sour migionette. You also get fresh made bread and breadsticks to go along with your oysters and beer. Their clam chowder is also delish- though a bit pricey at $12/bowl.
Friendly wait-staff too.