MBTA Subway Watch Boston This app provides real-time and accurate subway arrival info for Boston! Does not support the green line.
BostonBusMap Using GPS and info from MBTA this free app shows locations of buses in Boston and adjacent cities. It also predicts times on a map. Works great, updated frequently, nice UI. Recommend.
MBTA: AnyStop Whether you are in a rush to get your destination or if you want to see all your traveling options, AnyStop:MBTA is easy access to all T and buses line routes and schedules. Allows you to save stops/lines as "favorites"
MBTA Realtime T Boston Subway This app tracks MBTA trains in real time using sensors placed on the tracks. Get highly accurate train schedules with this app. Only Red, Orange, and Blue Lines - not Green yet.
Boston Map This free app has a Boston city map, subway map, and GPS and search. Maps available offline.