This app was mentioned in 3 comments, with an average of 3.00 upvotes
I'm a huge fan of MBTA Scout. It shows a map of all of your selected lines with the last reported location of all running vehicles. It's unfortunate that it has so few installs, because most other apps seem to emphasize estimated times. I've found those to be next to useless for buses.
For planning, Google Maps is excellent. I even recommend it over the T's own online planner. My only complaint about it is it doesn't tell you what the fare will be, but otherwise, it's great.
For live tracking, I use either MBTA Scout or NextBus. The T has their own live tracker on their site, as well. Some people swear by Transit, but for the Android app at least, the predictions are always a few minutes off.
I recently made MBTA Scout after finding other Android apps inadequate