Amon
A modern, self-hosted server monitoring system with system monitoring, process monitoring and notifications
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collectd
A daemon that collects system performance statistics periodically, and provides tons of plugins to store and analyze these values
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Glances
A cross-platform, curses-based monitoring tool written in Python
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Icinga
Icinga is an enterprise-grade monitoring system. It was forked from Nagios and has added many often new features and patches, including a HTML5 UI, REST interface, dynamic tables, etc. It is also backwards compatible with Nagios, so all the existing plugins can be reused.
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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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Sentry
Automate error tracking to find and fix issues before your users even experience a crash. Sentry is open-source, real-time, and loved by 100,000 developers around the world.
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Zabbix
An open source, enterprise-class monitoring framework
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