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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Nagios Core
Nagios Core network monitoring software is a powerful, enterprise-class host, server, application, monitoring and alerting tool
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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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