BitMeter OS is a free, open-source, bandwidth monitor that works on Windows, Linux and Mac OSX.
BitMeter OS keeps track of how much you use your internet/network connection, and allows you to view this information either via a web browser,
or by using the command line tools.
The Web Interface displays various graphs and charts that show how your internet/network connection has been used over time.
The Monitor pane displays a graph that updates once each second, showing you what’s happening with your connection right now: