From OpenNMS

OpenNMS has been designed from its inception to be enterprise-grade. For our purposes, this includes these features:

• High performance (vertical scaling?). A single instance of OpenNMS supports monitoring of a large number of nodes (tens of thousands? hundreds of thousands?) from a single server.
• Automation. OpenNMS minimizes the amount of manual configuration and reconfiguration needed by automatically performing tasks on a regular basis, like discovering new nodes, detecting services on new and existing nodes, and gathering response time and performance data.
• Rule-based configuration (policy-based?). Flexible rules can be used to specify what services are polled on certain devices, which SNMP data is collected and how often, and to whom various notifications should be delivered, without having to explicitly identify and configure each host in the monitoring system.