What are some good utilities for capturing packets?
For all flavors of Unix, tcpdump, available from http://www-nrg.ee.lbl.gov/, is the standard. tcpdump is a command-line tool, with no graphical user interface. Source code is included.
WinDump, available from http://netgroup-serv.polito.it/windump/, is a port of tcpdump for Windows. Source code is included.
Both of the Windows utilities require WinPCap, a packet capture driver for Windows available from http://netgroup-serv.polito.it/winpcap/.