__[ipac]__ / __ipac-ng__ add rules to the linux ipchains/iptables firewall. fwctl is an alternative for ipchains-based machines that does a similar thing.
__net-acct__ is a package that provides a user-mode daemon to sniff all ip traffic from a machine. This doesn't require the machine to be running an iptables firewall. Probably in your favourite distribution, otherwise http://exorsus.net/projects/net-acct/.
__netramet__ - quote from MattBrown: ''It was created to allow billing of traffic for the University of Auckland. It's very powerful but at the same time it appears quite complex to setup.'' http://www2.auckland.ac.nz/net/NeTraMet/.
__ntop__ (as in Network TOP). Displays summaries of network usage, either from the command-line in from a web-browser. In debian, or go to http://www.ntop.org/ntop.html