tcpdump

tcpdump is free Unix-based software that runs under the Unix/Linux command line. Some of the useful resources for it are:

• The tcpdump website: http://www.tcpdump.org/
• The Windows version of tcpdump (Windump): http://www.winpcap.org/windump/default.htm
• A friendly tutorial: http://danielmiessler.com/study/tcpdump/
• The official man page: http://www.tcpdump.org/tcpdump_man.html

Wireshark can open tcpdump files, so when you capture packets with tcpdump, you can later open it with Wireshark or any other graphical tool.