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You can use Networx to track your bandwith usage. Even though it doesn't let you see which site/program consume bandwith the most, it gives you the stats in real time and stores it up in hourly/weekly/monthly stats so you can figure it out on your own.

Fun fact about bandwith: Let's say there is a 3 Mb video on youtube that you like. Each time you go see it, you use 3Mb of bandwith in order to load it. The same amount as if you had downloaded it on your computer. Therefore, if you go see it 3 times, it will have been the same as if you had downloaded it 3 times. The more you know.

http://www.softperfect.com/products/networx/