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How do I find out which process is eating up my bandwidth?

Sometimes there is unusual internet activity on my Computer. The modem's lights are always blinking and when I open system monitor, I see there some unknown program is using my precious internet with 64KB/S (I have 512kbps connection). I am in a firefox session with only one tab opened, and the page is already loaded and there is no busy sign in the page (that rotating orange circle).

In that situation I unplug my modem and reconnect it again. After several times doing this, that unusual activity stops. It annoys me very much.

How can I find out the process which is using the Internet?. How much it is using? How can I kill it?

A graphical solution will be better.