Site Down - Site down outage and downtime reports from SiteDown.co
ServerSuit - ServerSuit is a browser based program that enables remote Linux administration, monitoring, website hosting, and server setup automation.
Is it Down? - Is it down can check if a website is down or is it just your PC that can not access it.
RedGate SQL Monitor - SQL Monitor helps you and your team find issues – before they become problems
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services