I'm not an expert in this field, so I just mention the tools I know. Please check also for other applications. Besides the usual tools tcpdump, netcat, there are two applications I prefer, which may be used to analyze network traffic:
The Multi Router Traffic Grapher (MRTG) is a tool to monitor the traffic load on network-links. MRTG generates HTML pages containing GIF images which provide a LIVE visual representation of this traffic. Check http://www.ee.ethz.ch/stats/mrtg/ for an example. MRTG is based on Perl and C and works under UNIX and Windows NT.
Network Top - ntop http://www-serra.unipi.it/~ntop/ is a UNIX tool 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 can be used in both interactive or web mode. In the first case, ntop displays the network status on the user's terminal. In web mode a web browser (e.g. netscape) can attach to ntop (that acts as a web server) 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.