IPAudit - Network Activity Monitor with Web Interface

IPAudit monitors network activity on a network by host, protocol and port. It listens to a network device in promiscuous mode, and records every connection between two ip addresses. A unique connection is determined by the ip addresses of the two machines, the protocol used between them, and the port numbers (if they are communicating via udp or tcp).

IPAudit can be used to monitor network activity for a variety of purposes. It has proved useful for monitoring intrusion detection, bandwith consumption and denial of service attacks. It can be used with IPAudit-Web to provide web based network reports.

IPAudit is a free network monitoring program available and extensible under the GNU GPL.
IPAudit is a command line tool that uses the libpcap library to listen to traffic and generate data. The IPAudit-Web package includes the IPAudit binary in addition to the web interface that creates reports based on the collected data. Using the Web package is recommended, as it gives you a slick graphical interface complete with traffic charts and a search feature.

You can download IPAudit here:

IPAudit 0.95 - Latest stable version of IPAudit
Or read more here.
You can also find a very good introduction to IPAudit by SecurityFocus here.