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Wireshark is the world's foremost network protocol analyzer. It lets you capture and interactively browse the traffic running on a computer network. It is the de facto (and often de jure) standard across many industries and educational institutions.

Fiddler is a Web Debugging Proxy which logs all HTTP(S) traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP(S) traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Fiddler includes a powerful event-based scripting subsystem, and can be extended using any .NET language.

tcpdump is a common packet analyzer that runs under the command line. It allows the user to intercept and display TCP/IP and other packets being transmitted or received over a network to which the computer is attached.

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.

NetHogs is a small 'net top' tool. Instead of breaking the traffic down per protocol or per subnet, like most tools do, it groups bandwidth by process. NetHogs does not rely on a special kernel module to be loaded. If there's suddenly a lot of network traffic, you can fire up NetHogs and immediately see which PID is causing this. This makes it easy to indentify programs that have gone wild and are suddenly taking up your bandwidth.

Microsoft Network Monitor is a packet analyzer. It enables capturing, viewing, and analyzing network data and deciphering network protocols. It can be used to troubleshoot network problems and applications on the network.

SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers. You can view the TCP/IP conversations in Ascii mode (for text-based protocols, like HTTP, SMTP, POP3 and FTP) or as hex dump (for non-text base protocols, like DNS). Application can capture TCP/IP packets on your network without installing a capture driver (works only for Windows 2000/XP or greater).

NetworkMiner is a Network Forensic Analysis Tool (NFAT) for Windows. NetworkMiner can be used as a passive network sniffer/packet capturing tool in order to detect operating systems, sessions, hostnames, open ports etc. without putting any traffic on the network. NetworkMiner can also parse PCAP files for off-line analysis and to regenerate/reassemble transmitted files and certificates from PCAP files.

Ethereal® is used by network professionals around the world for troubleshooting, analysis, software and protocol development, and education. It has all of the standard features you would expect in a protocol analyzer, and several features not seen in any other product. Its open source license allows talented experts in the networking community to add enhancements. It runs on all popular computing platforms, including Unix, Linux, and Windows.

Packetyzer provides a Windows user interface for the Ethereal packet capture and dissection library. Ethereal is used by network professionals around the world for troubleshooting, analysis, software and protocol development, and education. Ethereal has all of the standard features expected in a protocol analyzer, and several features not otherwise available.

iwScanner is a wireless scanner for linux with an easy to use graphic interface.

apptalk.ninja is a great, easy-to-use tool for app developers tackle that monitors and debugs issues with their apps’ communication over-the-air.

LANGuardian is the first and only deep packet inspection (DPI) software to provide SMB network managers with root cause information about network and user activity. LANGuardian gives a deeper level of detail than standard flow tools, and is an affordable alternative to premium appliance based DPI tools.

Packet Peeper is a free network protocol analyzer (packet sniffer) for Mac OS X.