collectd
A daemon that collects system performance statistics periodically, and provides tons of plugins to store and analyze these values
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ConDep
ConDep is a open source infrastructure configuration and deployment DSL (Domain Specific Language) specifically targeted to (but not limited to) the Windows Server platform. If your familiar with tools like Chef and Puppet, ConDep does very much the same, but with native support for Windows.
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ConfigCat
ConfigCat is a feature flags as a service. It gives you a web based dashboard to manage your feature flags + SDKs to integrate those feature flags into your applications. ConfigCat enables you to use the same feature flags in your mobile apps, websites and server-side applications. It offers all features for free. You get an enterprise-ready SLA with the pricing plans.
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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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Nagios Core
Nagios Core network monitoring software is a powerful, enterprise-class host, server, application, monitoring and alerting tool
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Zabbix
An open source, enterprise-class monitoring framework
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Zipkin
A distributed request tracking system, useful for micro-service and distributed architectures written in Scala
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