UptimeRobot - Free Website Uptime Monitoring
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.
RapidSpike - RapidSpike is a reliability, performance and security monitoring service for transactional websites. Using next-generation synthetic monitoring & deeper than ever before data, RapidSpike is for businesses serious about protecting their online revenue
UpTime.onl - Simple website uptime monitoring tool
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Better Uptime - We call you when your website goes down