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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Nagios Core
Nagios Core network monitoring software is a powerful, enterprise-class host, server, application, monitoring and alerting tool
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Prevoty Application Monitoring & Protection (AMP)
Prevoty enables DevOps to inject security into your web applications and web services. Protection is provided at runtime from within the application itself, and stays with the app regardless of deployment method (cloud, on-premises, etc). Applications are protected against threats including XSS, SQLi, CSRF, command injection, etc. Works with Puppet, Chef, Ansible, Docker, Splunk, QRadar, ELK, and many others.
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Statman
Statman makes it possible to instrument and collect statistics from your high-traffic production Erlang systems with very low overhead. The collected data points are aggregated in the VM and can be sent to services like Graphite, Munin, New Relic, etc.
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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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