Wireshark

Wireshark is an open source network protocol analyzer for both UNIX and Windows. It allows you to examine data from a live network or a capture file on disk.

For information and downloads, see http://www.wireshark.org/download.html.

Tcpdump/Windump

Tcpdump (and its Windows version, Windump) are network traffic analysis tools. These tools can be used to watch and diagnose network traffic according to various complex rules.

You can download Tcpdump from http://www.tcpdump.org.

You can download Windump from http://www.winpcap.org/windump.