collectd
A daemon that collects system performance statistics periodically, and provides tons of plugins to store and analyze these values
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Nagios Core
Nagios Core network monitoring software is a powerful, enterprise-class host, server, application, monitoring and alerting tool
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Rex
A agent-less server orchestration tool built on Perl
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Statman
Statman makes it possible to instrument and collect statistics from your high-traffic production Erlang systems with very low overhead. The collected data points are aggregated in the VM and can be sent to services like Graphite, Munin, New Relic, etc.
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Zabbix
An open source, enterprise-class monitoring framework
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