UptimeRobot - Free Website Uptime Monitoring
WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Oh Dear! - Website monitoring, without the hassle
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.
Freshworks - Delight your customers, from wherever you are