Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
ServerSuit - ServerSuit is a browser based program that enables remote Linux administration, monitoring, website hosting, and server setup automation.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Better Uptime - We call you when your website goes down
RedGate SQL Monitor - SQL Monitor helps you and your team find issues – before they become problems