OpenNMS, "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." (from opennms.org)
100% Free and Open Source SoftwareEdit
There is only one version of OpenNMS, freely available under the GPLv2 and including all features. Contrast this to the recent profusion of fauxpen source projects in the same space.