10 Killer Open Source Networking Tools
#5 OpenNMS and Cacti
OpenNMS is a highly scalable network monitoring system that is completely open source software. A single server can monitor hundreds of thousands of network interfaces and produce nice graphs for metrics such as bandwidth usage, CPU, memory, and more.
You can set thresholds that indicate when a device is busy or down and receive a notification via email, SMS, IM, and so on. And you can also have separate logins for each of your NOC team, and you can set up an on-call schedule so that notifications go only to on-duty team members. OpenNMS also has an escalation handler, so if the level-one NOC techs don't take care of an issue right away, an engineer or manager can be notified to oversee issue resolution.
The Cacti graphing solution makes a good complement to OpenNMS. Although OpenNMS has the same graphing capabilities, Cacti's more intuitive Web UI allows nontechnical staff to build and manage collections of graphs that are interesting to them.