Nagios Core™ is the monitoring and alerting engine that serves as the primary application around which hundreds of Nagios projects are built.
Nagios Core Overview
Nagios Core has been designed with a focused, extensible architecture that is designed for flexibility and scalability. It provides several APIs to allow its feature-set to be easily extended through additional addons. This architecture has proved to be successful and has spawned the creation of thousands of addon projects that extend its core feature set.