Zabbix - Track, record, alert and visualize performance and availability of IT resources
TrafficMonitor - TrafficMonitor is a network monitoring suspension window software in Windows.
Observium - Observium
NetBalancer - NetBalancer is a Windows application for local network traffic control and monitoring.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
FreeMeter - Monitor network bandwidth (C#.NET 2k/XP+). Desktop and Systray graph.