TCP/DUMP

Tcpdump is the network sniffer we all used before (Wireshark) came on the scene, and many of us continue to use it frequently. It may not have the bells and whistles (such as a pretty GUI and parsing logic for hundreds of application protocols) that Wireshark has, but it does the job well and with less security risk. It also requires fewer system resources. While Tcpdump doesn't receive new features often, it is actively maintained to fix bugs and portability problems. It is great for tracking down network problems or monitoring activity. There is a separate Windows port named WinDump. tcpdump is the source of the Libpcap/WinPcap packet capture library, which is used by Nmap and many other tools.

Cain and Abel

UNIX users often smugly assert that the best free security tools support their platform first, and Windows ports are often an afterthought. They are usually right, but Cain & Abel is a glaring exception. This Windows-only password recovery tool handles an enormous variety of tasks. It can recover passwords by sniffing the network, cracking encrypted passwords using dictionary, brute-force and cryptanalysis attacks, recording VoIP conversations, decoding scrambled passwords, revealing password boxes, uncovering cached passwords and analyzing routing protocols.

Wire Shark

Wireshark (known as Ethereal until a trademark dispute in Summer 2006) is a fantastic open source multi-platform network protocol analyzer. It allows you to examine data from a live network or from a capture file on disk. You can interactively browse the capture data, delving down into just the level of packet detail you need. Wireshark has several powerful features, including a rich display filter language and the ability to view the reconstructed stream of a TCP session. It also supports hundreds of protocols and media types. A tcpdump-like console version named tshark is included. One word of caution is that Wireshark has suffered from dozens of remotely exploitable security holes, so stay up-to-date and be wary of running it on untrusted or hostile networks (such as security conferences).