Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
SolarWinds Network Configuration Manager - SolarWinds is a leading provider of powerful and affordable IT infrastructure management software. Make IT look easy.
netcat - Netcat is a featured networking utility which reads and writes data across network connections...
Cisco Network Assistant - Cisco Network Assistant simplifies wired and wireless network management, and provides critical product information using Cisco Active Advisor.
Ettercap - Ettercap is a suite for man in the middle attacks on LAN.