Darkstat.

I installed Darkstat today, a packet sniffer which runs as a background process and displays its output as a web page in your browser. It installed with sudo apt-get install darkstat and gives a warning that it needs to be configured as part of its installation process. This is done at /etc/darkstat/init.cfg and won’t run until it is configured. Its started by running 'sudo /etc/init.d/darkstat start' at the terminal window.

And here all of the web pages and man page are wrong! You view the results in your browser by showing http://localhost:667/ and NOT port 666 like it says in the documentation. When it is running it doesn't show in top, and its database doesn't show anywhere by various methods of searching for it!

I feel that it is only slightly useful, but that vnstat is more useful giving details of your bandwidth usage.