M/Monit - Monit is a free open source utility for managing and monitoring, processes, files, directories and filesystems on a UNIX system. Monit conducts automatic maintenance and repair and can execute meaningful causal actions in error situations.
FreeMeter - Monitor network bandwidth (C#.NET 2k/XP+). Desktop and Systray graph.
Lansweeper - Lansweeper discovers Windows, Linux and Mac machines in your network.
NetSpeedMonitor - NetSpeedMonitor is a lightweight Network Monitoring Toolbar for your Windows Taskbar
Zabbix - Track, record, alert and visualize performance and availability of IT resources
TrafficMonitor - TrafficMonitor is a network monitoring suspension window software in Windows.