ServerSuit - ServerSuit is a browser based program that enables remote Linux administration, monitoring, website hosting, and server setup automation.

NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.

Nagios - Complete monitoring and alerting for servers, switches, applications, and services

SolarWinds System Management - Performance monitoring and management for applications, Websites, servers, storage arrays and virtual infrastructure.

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.

Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.