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.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
Ettercap - Ettercap is a suite for man in the middle attacks on LAN.
Advanced IP Scanner - Advanced IP Scanner shows all network devices, gives you access to shared folders, and can even remotely switch computers off. Download it Free.
tcpdump - tcpdump is a common packet analyzer that runs under the command line.