Ethereal
http://www.ethereal.com
libpcap
http://www.tcpdump.org
libpcap tutorial
http://www.cet.nau.edu/~mc8/Socket/Tutorials/section1.html
Packetyzer
http://www.networkchemistry.com/products/packetyzer/
Pcap tutorial
http://www.tcpdump.org/pcap.htm
Sans TCP/IP Guide
http://www.sans.org/resources/tcpip.pdf
tcpdump
Tethereal
WinDump
http://windump.polito.it
WinPcap
http://winpcap.polito.it