WebGazer is an uptime monitoring platform that checks if your website is up as it should be and notifies you if anything goes wrong
Site Down - Site down outage and downtime reports from SiteDown.co
UptimeRobot - Free Website Uptime Monitoring
Is it up? - A simple tool to check if a website or ip address is up or down.
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.
downinspector.com - Find out if a website is down for everyone or just you.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring