I like El Charro at 4 Charles Street. Not all the entrees are under $20, but most are, and the food is fab. As well, if you ask, they'll make any entree as a tapa for you.
Extra Virgin, nice atmosphere and tasty drinks. Da Andrea for Italian (tho there's been some discussion on this board favoring Maletesta, but I can't comment since I haven't been to the latter). If you're into sharing small plates you could go the route of Alta.