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.
TrafficMonitor - TrafficMonitor is a network monitoring suspension window software in Windows.
NetMonster - NetMonster is a networking app by Michal Mrocek that comes up with features to enable you to get detailed information about your current network right on your mobile phone screen.
NetBalancer - NetBalancer is a Windows application for local network traffic control and monitoring.
tcpdump - tcpdump is a common packet analyzer that runs under the command line.
FreeMeter - Monitor network bandwidth (C#.NET 2k/XP+). Desktop and Systray graph.