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 - online demo available.
The Monitor pane displays a graph that updates once each second, showing you what's happening with your connection.
BitMeter OS also lets you define Alerts to notify you when your internet usage exceeds some limit that you specify.