If you are on one of those internet connections, which have limited monthly bandwidth and once bandwidth limit is reached, your ISP starts charging some amount per MB, then Net Meter is the tool for you.

Net Meter is a freeware utility that sits in your system tray and constantly monitors your network and shows a graph of your network usage in real time in a pop up window.

Under Notification tab in Options Window, you can make a setting that Net Meter alerts you once your monthly limit is reached.

Besides, you can also customize the graph display,  set Net Meter to load at system start up and lots more.

Download Net Meter