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Naemon
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Shinken
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Check MK
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Icinga
EXPLANATION
Nagios Core is a free and open source computer-software
application that monitors systems, networks and infrastructure,
initially released in 1999.
Icinga is an open source computer system and network
monitoring application. It was originally created as a fork of the
Nagios system monitoring application in 2009.
Naemon is an open source computer system monitoring, network
monitoring and infrastructure monitoring software application. Initial
release 2015 and based on Nagios 4 core.
Shinken is an open source computer system and network monitoring software application. Released as a fork of Nagios in 2009.
Check_MK is an extension to the Nagios monitoring system that allows
creating rule-based configuration using Python and offloading work from
the Nagios core to make it scale better, allowing more systems to be
monitored from a single Nagios server. Initial release 2008.
SOURCE
https://en.wikipedia.org/wiki/Nagios#See_also