People often want to know how to determine their IP usage, especially since ADSL accounts in NewZealand have data caps on them. People with LANs often want to know how much traffic each machine is using, etc.
Here are some brief pointers:
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/.
netacct-mysql is a fork of the above net-acct - using mysql as it's database backend. A very nice piece of software, with the ability to distinguish (when given the appropriate routes) between local, national, and international traffic. Found at http://netacct-mysql.gabrovo.com/.
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
trafd is a portable traffic accounting daemon built on top of libpcap. In FreeBSD ports and at http://www.riss-telecom.ru/dev/trafd/ (which also has a bunch of links to other packages).