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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.

Scapy is a powerful interactive packet manipulation program. It is able to forge or decode packets of a wide number of protocols, send them on the wire, capture them, match requests and replies, and much more. It can easily handle most classical tasks like scanning, tracerouting, probing, unit tests, attacks or network discovery (it can replace hping, 85% of nmap, arpspoof, arp-sk, arping, tcpdump, tethereal, p0f, etc.). It also performs very well at a lot of other specific tasks that most other tools can't handle, like sending invalid frames, injecting your own 802.11 frames, combining technics (VLAN hopping+ARP cache poisoning, VOIP decoding on WEP encrypted channel, ...), etc. »

Qcheck, the network troubleshooting utility from Ixia, quickly checks network response time, throughput, and streaming performance. It even runs anywhere-to-anywhere traceroute!

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.

IxChariot is the industrys leading test tool for simulating real-world applications to predict device and system performance under realistic load conditions. Comprised of the IxChariot Console, Performance Endpoints and IxProfile, the IxChariot product family offers thorough network performance assessment and device testing by simulating hundreds of protocols across thousands of network endpoints. IxChariot provides the ability to confidently assess the performance characteristics of any application running on wired and wireless networks »

packeth is GUI and CLI packet generator tool for ethernet. Primary it is developed and maintained for Linux, but some ports for Windows and MAC have also be done. It allows you to create and send any possible packet or sequence of packets on the ethernet link. Main advantages are that is really simple to use, powerful and supports many custom features. And lastly, it has the most beautiful web site of all the packets generators.