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
Site Down - Site down outage and downtime reports from SiteDown.co
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Is it Down? - Is it down can check if a website is down or is it just your PC that can not access it.
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.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.