ntop is a network traffic probe that shows the network usage, similar to what the popular top Unix command does. ntop is based on libpcap and it has been written in a portable way in order to virtually run on every Unix platform and on Win32 as well.
ntop users can use a a web browser (e.g. netscape) to navigate through ntop (that acts as a web server) traffic information and get a dump of the network status. In the latter case, ntop can be seen as a simple RMON-like agent with an embedded web interface. The use of:
a web interface
limited configuration and administration via the web interface
reduced CPU and memory usage (they vary according to network size and traffic)
make ntop easy to use and suitable for monitoring various kind of networks.
Read more at ntop.org