Prometheus - An open-source systems monitoring and alerting toolkit.
ServerSuit - ServerSuit is a browser based program that enables remote Linux administration, monitoring, website hosting, and server setup automation.
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
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
StatsD - Simple daemon for easy stats aggregation.