[ Thanks to linuxaria for this link.
“In my view any product in this category must deal with Nagios,
this project was born in 1996 and over time has established itself
as one of the best open source monitoring projects.
“The project is divided into Nagios Core and and the paid
version with support Nagios XI that has some additional features.
Nagios Core is the monitoring and alerting engine that serves as
the primary application around which hundreds of Nagios projects
are built. It serves as the basic event scheduler, event processor,
and alert manager for elements that are monitored. It features
several APIs that are used to extend its capabilities to perform
additional tasks, is implemented as a daemon written in C for
performance reasons, and is designed to run natively on Linux/*nix