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.
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StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Is it up? - A simple tool to check if a website or ip address is up or down.
Downtime Monkey - Downtime Monkey: website downtime alerts & uptime monitoring worldwide.