OpenNMS is the world's first enterprise grade network management application platform developed under the open source model.
Well, what does that mean?
The OpenNMS Project was started in July of 1999 and registered on SourceForge in March of 2000. It has years of experience on the alternatives.
It was designed from "day one" to monitor tens of thousands to ultimately unlimited devices with a single instance. It brings the power, scalability and flexibility that enterprises and carriers demand.
While OpenNMS is useful "out of the box," it is designed to be highly customizable to create an unique and integrated management solution.
OpenNMS is 100% Free and Open Source software, with no license fees, software subscriptions or special "enterprise" versions.
For details on installing OpenNMS, please see the install guide: Installing OpenNMS.
For details on building OpenNMS, please see the wiki page: Building OpenNMS.