collectd

A daemon that collects system performance statistics periodically, and provides tons of plugins to store and analyze these values

     

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Monit

An open source utility for managing and monitoring processes, programs, files, directories and filesystems

     

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SignalFx

SignalFx is an advanced monitoring and alerting solution for modern infrastructure. With SignalFx, development and operations teams monitor, analyze, and alert in real-time on any time series data.

     

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Zabbix

An open source, enterprise-class monitoring framework

     

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