Zabbix - Track, record, alert and visualize performance and availability of IT resources
FreeMeter - Monitor network bandwidth (C#.NET 2k/XP+). Desktop and Systray graph.
Paessler PRTG - An advanced network monitoring solution to monitor network up/downtime, traffic and usage.
TrafficMonitor - TrafficMonitor is a network monitoring suspension window software in Windows.
Observium - Observium
NetSpeedMonitor - NetSpeedMonitor is a lightweight Network Monitoring Toolbar for your Windows Taskbar