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.
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.
Winsock Packet Editor - WPE Pro is a packet editor.
NetworkMiner - NetworkMiner is a Network Forensic Analysis Tool (NFAT) for Windows.
Scapy - Scapy is a powerful interactive packet manipulation program.