netcat!

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.

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).

Debookee is the simplest & most powerful network traffic analyzer for macOS.

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.

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.It supports active and passive dissection of many protocols (even ciphered ones) and includes many feature for network and host analysis.

socat is a relay for bidirectional data transfer between two independent data channels. Each of these data channels may be a file, pipe, device (serial line etc. or a pseudo terminal), a socket (UNIX, IP4, IP6 - raw, UDP, TCP), an SSL socket, proxy CONNECT connection, a file descriptor (stdin etc.), the GNU line editor (readline), a program, or a combination of two of these. These modes include generation of "listening" sockets, named pipes, and pseudo terminals.

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.

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.

TcpLogView is a simple utility that monitors the opened TCP connections on your system, and adds a new log line every time that a TCP connection is opened or closed.

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.

PsPing is a command-line utility for measuring network performance. In addition to standard ICMP ping functionality, it can report the latency of connecting to TCP ports, the latency of TCP and UDP round-trip communication between systems, and the TCP or UDP bandwidth available to a connection between systems. Besides obtaining min, max, and average values in 0.01ms resolution, you can also use PsPing to generate histograms of the results that are easy to import into spreadsheets.

tcpflow, a TCP Flow Recorder, is a program that captures data transmitted as part of TCP connections (flows), and stores the data in a way that is convenient for protocol analysis or debugging. A program like 'tcpdump' shows a summary of packets seen on the wire, but usually doesn't store the data that's actually being transmitted. In contrast, tcpflow reconstructs the actual data streams and stores each flow in a separate file for later analysis.

Pirni Pro is a network security tool designed for iOS, and specifically for iPhone and iPod Touch devices. It is capable of intercepting traffic on a wireless network segment, capturing passwords, and regular expressions entered by the user.The core system of Pirni, written in C, is open-source software, and licensed under the GNU General Public License. Pirni Pro is the succeeding version of Pirni, and is commercial software, available in the Cydia Store, for jailbroken Apple devices.FeaturesPirni Pro supports active dissection of all non-ciphered protocols (given that the user has supplied a regular expression for dissection). The application description contains the following: ARP spoof the entire network or any target Watch a live feed over interesting packets collected Manage regular expressions to filter out data, such as site credentials In addition, the software also offers the following features: Password collectors for: HTTPExternal links Root at Everything official website Cydia Link Extensive Tutorial »

PPerf - PowerShell Iperf GUI

AirSnare monitors network traffic for unfriendly MAC addresses and alerts you when a MAC address is found that isn't on the friendly list. AirSnare also monitor DHCP requests from clients.