Zabbix - Track, record, alert and visualize performance and availability of IT resources
Opsmatic - Opsmatic offers complete system visibility and configuration monitoring for fast-moving technical teams.
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.
Prometheus - An open-source systems monitoring and alerting toolkit.
Guacamole - Access your computers from anywhere. Because the Guacamole client is an HTML5 web application, use of your computers is not tied to any one device or location. As long as you have access to a web browser, you have access to your machines.
Grafana - Data visualization & Monitoring with support for Graphite, InfluxDB, Prometheus, Elasticsearch and many more databases