The best protocol analyzer is Wireshark (formerly Etherreal), and it is free. Get it here.
A great "man in the middle" tool: Ettercap. Ettercap is a suite for man in the middle attacks on LAN. It features sniffing of live connections, content filtering on the fly and many other interesting tricks. Can be found here.
Snort® is an open source network intrusion prevention and detection system (IDS/IPS) developed by Sourcefire. Combining the benefits of signature, protocol, and anomaly-based inspection, Snort is the most widely deployed IDS/IPS technology worldwide. You can download Snort here.
Netcat is a featured networking utility which reads and writes data across network connections, using the TCP/IP protocol. It is designed to be a reliable "back-end" tool that can be used directly or easily driven by other programs and scripts. At the same time, it is a feature-rich network debugging and exploration tool, since it can create almost any kind of connection you would need and has several interesting built-in capabilities. Get more info and download it here.
Network Stumbler is a free Windows 802.11 (wireless) Sniffer. This tool finds open wireless access points, or “wardriving”, as it is most often referred to. Find it here.
AirSnort is an 802.11 WEP encryption cracking tool for local area networks. AirSnort operates by passively monitoring transmissions, computing the encryption key when enough packets have been gathered. You can get it here.
Other Tools and Information
The Sysinternals web site was created in 1996 by Mark Russinovich and Bryce Cogswell to host their advanced system utilities and technical information. Whether you’re an IT Pro or a developer, you’ll find Sysinternals utilities to help you manage, troubleshoot and diagnose your Windows systems and applications. Click here to go to the web page.