I heart Gin Mill.
It is an unassuming we're not serving you food lets play darts kind of place. They have a big screen for watching games and a pretty decent DJ on the weekends. Drinks aren't too expensive and everyone there seems pretty friendly. After a few too many drinks one night I realized they have a popcorn machine with free popcorn. I may or may not have had multiple bowls and began randomly sharing with strangers. There's good specials and an abundance of bar games (the only sports I excel at) including 3 dart boards, shuffleboard, some arcade games, and cornhole on the roof.
Check it out if you're in the South End!