UptimeRobot - Free Website Uptime Monitoring
Downtime Monkey - Downtime Monkey: website downtime alerts & uptime monitoring worldwide.
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.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
HetrixTools - FREE Uptime Monitoring + Server Monitoring + Status Page + Blacklist Monitoring!