WebGazer is an uptime monitoring platform that checks if your website is up as it should be and notifies you if anything goes wrong
Server Monitoring - A server monitor is a tool that monitors your server and website on close intervals and reports to you if it's inaccessible.
UptimeRobot - Free Website Uptime Monitoring
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
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.
Platypus - Realtime server monitoring and reporting
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring