collectd
A daemon that collects system performance statistics periodically, and provides tons of plugins to store and analyze these values
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Errbit
An open source, self-hosted error catching tool, which can collect and manage errors from other applications. It is compatible with the Airbrake API and can be used as a drop-in replacement.
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FastForward
Flexible operating-system event and metric forwarding agent. Receives input and forwards output using plugins.
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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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Puppet Dashboard
A web interface for Puppet. It can view and analyze Puppet reports, assign Puppet classes and parameters to nodes, view inventory data and backed-up file contents, etc.
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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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