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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FnordMetric
A HTML5/Javascript API for collecting and visualizing time series data
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Fog
The Ruby Cloud API library. Provides a Ruby API for interacting and managing many cloud providers, such as spinning up new instances, terminating old ones, attaching storage, etc. Currently supports more than 20 providers, including EC2, Rackspace, Brightbox, etc.
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Monigusto
A collection of Chef recipes to install various monitoring tools (such as sensu, statsd, etc). Makes it a breeze to get a monitoring infrastructure up and running
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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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testssl.sh
A command-line tool which checks a server's service on any port for the support of TLS/SSL ciphers, protocols as well as some cryptographic flaws. It's designed to provide clear output for your "is this good or bad" decision.
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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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