Free tools and utilities you might actually use to perform packet sniffing, to help you identify network issues and bottlenecks. These tools and utilities you can use to monitor and capture individual packets on your network to troubleshoot network problems or to find out what an attacker might see.

Ethereal Network Analyzer
Ethereal is a free network protocol analyzer for Unix and Windows. It allows you to examine data from a live network or from a capture file on disk. You can interactively browse the capture data, viewing summary and detail information for each packet. Ethereal has several powerful features, including a rich display filter language and the ability to view the reconstructed stream of a TCP session.

Snoop Analyzer Standard

SnoopAnalyzer Standard is a network protocol analyzer based on network data capturing technology under Microsoft Windows platforms(95/98/Me/2000/NT/XP).  

AnalogX PacketMon

AnalogX PacketMon allows you to capture IP packets that pass through your network interface - whether they originated from the machine on which PacketMon is installed, or a completely different machine on your network!

Network Probe

This free network monitor and protocol analyzer gives you an instant picture of the traffic situation on your network and enables you to monitor network traffic in real time, hunt down, identify, and isolate traffic problems and congestions on your network.

Analyzer
Analyzer is a full configurable network analyzer program for Win32 environment. Analyzer is able to capture packets on all platforms (and link-layer technologies) supported by WinPcap, except for Windows 95.

Sniphere
Sniphere is an another network wiretapping program for Windows using winpcap. Nevertheless, Sniphere is a pretty handy program with a lot of possibilities which most of free sniffers do not have.

Network Stumbler Wireless Packet Sniffer

Network Stumbler is a neat little program that allows you to see all the available Wi-Fi access points and networks that are in range of you and your active Wi-Fi computer. Network Stumbler is for desktop and laptop machines, and Mini Stumbler is the equivalent for the Wi-Fi enabled PDA.  

IP Sniffer 

IP sniffer is a popular freeware packet sniffer that uses the XP/2K Raw Socket features. IP Sniffer supports filtering rules, adapter selection, packet decoding, advanced protocol description and more. Detailed information about each packet is provided in a tree-style view, and the right-click menu allows to resolve or scan the selected source IP address. There are many additional features that are very useful.

PlasticSniffer
PlasticSniffer is a small and easy packet sniffer. In spite of its small size, PlasticSniffer offers some advanced features like IP filtering, port specific tracking, IP list and and option to ignore selected IPs. It does not offer the capabilities of a full featured packet sniffer, but it is exceptionally easy to use, and provides the information the average user is interested in. You must have .NET framework installed for PlasticSniffer to work.