ServerSuit - ServerSuit is a browser based program that enables remote Linux administration, monitoring, website hosting, and server setup automation.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
AppZero - AppZero is a monitoring and migration tool that allows users to keep track of different applications and servers in both simple and complex IT environments.
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.
RedGate SQL Monitor - SQL Monitor helps you and your team find issues – before they become problems
Prometheus - An open-source systems monitoring and alerting toolkit.