Zabbix is an open source network monitoring tool that works with a centralized Linux-based Zabbix server.
The Zabbix server automatically collects and parses data from monitored hardware so administrators can check availability and see trends in network performance. Zabbix can send out alerts and notifications if specific predefined events are occurring. It can also provide graphing functionality, which allows an administrator to get an overview of the current state of specific nodes and the network in general.
Some of Zabbix's other features include automatic discovery for network hosts and devices, a web-based front end that allows full system control from a browser, alerts, visual maps that show all the devices and hosts connected to the network and multi-database support.
This video provides a quick look at the Zabbix user interface.