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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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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