From OpenNMS

OpenNMS is the world's first enterprise grade network management platform developed under the open source model. It consists of a community supported open-source project as well as a commercial services, training and support organization.

As a network management platform, it provides several major feature categories:

1. Discovery - finding out what equipment is on your network, and adding it to the database
2. Monitoring - keeping track of the status of networked equipment
3. Events - receiving, correlation and sending notifications
4. Data collection - collection, storage, and reporting of various network available data points.