SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
tcpdump - tcpdump is a common packet analyzer that runs under the command line.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
Packet Sender - Packet Sender can send and receive UDP, TCP, and SSL on the ports of your choosing.
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.