Zabbix - Track, record, alert and visualize performance and availability of IT resources
ServerSuit - ServerSuit is a browser based program that enables remote Linux administration, monitoring, website hosting, and server setup automation.
Observium - Observium
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.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
RedGate SQL Monitor - SQL Monitor helps you and your team find issues – before they become problems