WhoISP

Monitoring and SNMP
big brother system and network monitor - high availability web-based monitoring notification & reporting. Big Brother License
MRTG - , MRTG stands for Multiple-Router Traffic Graph, a utility that allows you to monitor and graph just about any SNMP variable you can provide.
NetSaint Network Monitor - NetSaint is a program that will monitor hosts and services on your network. GPL
pacmon - If you need to accurately measure bandwidth used by the different hosts on your TCP/IP Ethernet network, then PacMon is the tool you've been waiting for.(commercial)
PACT - Port Accounting and Collection Tool - PACT - is an SNMP-Tool to do port-accounting for any device that conforms to SNMP standards and has at least a certain amount of SNMP-querieable data (e.g. octets in/out, etc.).
RDDtool - RRDtool (Round Robin Database tool) is a system to store and display time-series data.
VNC
What's Up Gold - WhatsUp Gold is an easy to use network monitoring, alerting, and recovery solution. Commercial.