FreeMeter - Monitor network bandwidth (C#.NET 2k/XP+). Desktop and Systray graph.
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.
Network Lights - Network Lights is a Windows program, which blinks keyboard LEDs (Light Emitting Diode) indicating outgoing and incoming network packets on network interface.
tcpdump - tcpdump is a common packet analyzer that runs under the command line.
TrafficMonitor - TrafficMonitor is a network monitoring suspension window software in Windows.
NetworkMiner - NetworkMiner is a Network Forensic Analysis Tool (NFAT) for Windows.