ZDNet's Computer Shopper Network Utilities
Packetstorm's packet sniffer section with over 100 sniffing programs
Wild Packets, makers of EtherPeek
EtherPeek
TamoSoft, makers of CommView
Opt Out is a great program worth looking at and the site has a wealth of information about packet sniffing
An overview of the TCP/IP Protocol Suite
Version (4) Internet Header Length (4) Type of Service (8) Total Length (16)
Identification (16) Flags (3) Fragment Offset (13)
Time To Live (8) Protocol (8) Header checksum (16)
Source Address (32)
Destination Address (32)
Options (Variable) Padding (0-24)