WebGazer is an uptime monitoring platform that checks if your website is up as it should be and notifies you if anything goes wrong
UptimeRobot - Free Website Uptime Monitoring
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Monkey Test It - Primate powered algorithms that scan your site for common bugs and SEO mistakes.
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.
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.