Zabbix - Track, record, alert and visualize performance and availability of IT resources
facileManager - facileManager is a modular suite of web apps built with the system administrator in mind. Say good-bye to manual management of services running on multiple servers.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
Atomia DNS - Atomia DNS is a free & open source DNS management system.
Prometheus - An open-source systems monitoring and alerting toolkit.
Webmin - Webmin is a web-based interface for system administration for Unix.