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.
AppBeat - AppBeat is reliable website monitoring service that alerts you when there is a downtime or failure. It also measures performance and provides functional tests.
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 Doctor - You can monitor up to 5 URLs with 1-minute monitoring interval, for FREE!
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring