The Netlimiter plugin by EquilibriuM.
Netlimiter is a very discreet program and it's very hard to extract data from it. The plugin takes its data from netlimiter's tray icon popup text, therefore you must enable the Tray Icon from Netlimiter settings.
Tested with XP SP2 and Netlimiter 2 pro (I'm very sure that this won't run on Vista x32 or x64). You also need dotnet framework 3.0 (it has very nice string handling so I couldn't abide the crappy std::string controls).