ntop is a Unix utility that displays network usage. It runs on nearly any Unix platform, as well as Windows. Ntop features an embedded web server that provides VERY useful information about, among many other things, traffic distribution, packet sizes, protocol distribution, and active TCP sessions, using graphs and charts. (I use it personally and have found it to be extremely helpful) New versions of the utility also include a network shell called intop that presents the data in a similar fasion to the Unix top utility.
ntop is available at