Ethereal

Ethereal is a network protocol analyzer for Unix and Windows. It allows you to examine data from a live network or from a capture file on disk. You can interactively browse the capture data, viewing summary and detail information for each packet.

You will have to have WinPCap for this too
http://www.mirrors.wiretapped.net/security/packet-capture/winpcap/

I was able to locate the packets flowing in the network and guess can be very useful if you want to sniff around what others are sending across in the network. I tried to see if I can find out the password to the unix machine I telnet to. I wasnt able to do so. Am I doing something wrong? I was able to find the packets related to my telnet session but a string search for my password did not find it!