This page lists useful tools for network management. See also NetworkTroubleshooting.
A SNMP network monitoring system that knows about various device types, and can autoconfigure itself for devices
This tool generates pretty graphs of your network from rrd sources showing quickly at a glance as to how loaded your network is.
Yet another SNMP monitoring system
See TrafficAccounting for details
An open source IDS
Nagios is a tool that warns you when services/hosts enter a bad state.
Nessus is a tool that checks hosts for vunerabilities.
Nmap ("Network Mapper") is a free open source utility for network exploration or security auditing.