Free packet sniffer software for windows

Packet Spy is a free and useful packet sniffer utility.
Packet Spy is a tool designed with the purpose of monitoring incoming and outgoing packets from a given network. It is most useful in situations where you need to verify the security of a packet or monitor the responses invoked by the network.
It features an onscreen view of sent/received packets and can also log results to a file for further examination.
Main features:
- Displays incoming and outgoing packets from a given source
- Create filters to monitor for specific traffic
- Output reports to log files for further examination
- Displays IP/TCP/UDP packet header information

Intercepts and decodes all instant message traffic sent/received by the computer (currently AOL Instant Messenger, ICQ and MSN but will support Yahoo and jabber by version 0.8). A high performance engine delivers real time message logging. File/picture transfers can also be intercepted. Conversations can be viewed immediately or saved for later analysis. This program is intended for concerned parents/spouses as well as network administrators.
Open-Source Freeware

Connection Sniffer is a free application that was created to help determine who was connecting to the machine and get a general idea of what was being sent. Not only will it tell you how many packets are going in and out, but also grab the text that it is sending or receiving.
This was mostly designed for trouble shooting machines that are not server based, since Windows 2000 does have a Network Monitor that will do the same thing.

Sniffer.NET 1 is an advanced program which satisfies you because it can track all network traffic.

Two times PlanetSourceCode.com Contest winner. Sniffer.NET is an open source Network Packet Capture and Network Monitoring Tool in VB.NET. It utilizes the power of Packmon.NET Library written in VB.NET which can monitor all network traffic. Now Supports User plugins which can be written in any .Net Language till they expose the IPlugin interface. Sample Plugin is also provided with the Source.

PacketMon is an easy to use packet sniffer with advanced rules for filter, CSV output, streaming to a file and more...
Are you interested in not just knowing that a particular program is using the internet, but actually viewing the communications? Do you need to log certain network activity, but have no easy means to do so? Or are you just bored and looking for a new network toy to play with? Whether youre a network administrator, network programmer or power user, AnalogX PacketMon is a fast and simple to use network monitor!
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! Once the packet is received, you can use the built in viewer to examine the header as well as the contents, and you can even export the results into a standard comma-delimited file to importing into your favorite program. As if thats not enough, PacketMon has a powerful rule system that allows you to narrow down the packets it captures to ensure you get EXACTLY what youre after, without tons of unrelated information.