Network usage per app

Hello,

I'm looking for an app or utility (command line) which can measure sent/received amount of data of running apps. When I'm on a train or a bus, I sometimes am limited by the amount of data I can use up. I know that I could use Activity Monitor but it's really not comfortable to use. Also I'd have to remember to open the app when I am on a limited connection. I couldn't find anything usable on the net.