Macos – How to monitor network usage by process on Mac OS X?

displaymacosnetworkingtraffic

Is there any way to find out which process using how much internet bandwidth on Mac OS X Lion?
I'm on mobile internet now, which is not very fast, so it would be nice if I can tell that for example, Chrome using 10kB/s, and Skype using 2kB/s.

I can see the total amount of traffic in Activity Monitor, but it is not enough for me.

I'd like to use an existing application, not interested to write an app like this.
And I'm not interested in the actual traffic, only the number of bytes transferred and received by each processes.

Mac OS X Lion (10.7.x) contains a command called nettop that gets you most of the way there. Be sure to check the man page to learn the navigation keys and note the d key for toggling delta display. Not quite the kB/s display you were looking for, but close perhaps.
nettop has lots of columns that it won't draw and won't even tell you about if your terminal window is too narrow.