collectd
A daemon that collects system performance statistics periodically, and provides tons of plugins to store and analyze these values
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Instrumental
Instrumental is a real-time, high-scale application and server monitoring tool with intelligent alerts, a powerful query language, and free development accounts.
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Pallet
A tool to provision and maintain servers across various cloud platforms as well as virtual machine platforms, without any dependencies. Provides cloud and operating system independence.
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Zabbix
An open source, enterprise-class monitoring framework
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