OpenNMS Meridian and Horizon are both fully open source and licensed under the AGPLv3. What's the difference?
Meridian is an optimized, stable version of OpenNMS, via a subscription, that maximizes the platform’s value and minimizes the effort required to maintain it. Meridian is curated by The OpenNMS Group for production environments. Matured features from Horizon are incorporated into an annual major Meridian release and updates are designed to be much easier for Meridian.
Horizon is available to download from our repository on Github for free and releases early and often. Focused on innovation, Horizon contains the latest updates to OpenNMS; however, not all features may be production-ready.