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.
Pingometer - Pingometer is a service that monitors the uptime, downtime, and performance of websites. Get 24/7 monitoring - sign up for your FREE account today!
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Site24x7 - Site24x7 offers both free & paid website monitoring services. Monitor websites remotely and receive instant email/sms alerts if your website becomes unavailable. View uptime & performance graphs of your website monitors.
HetrixTools - FREE Uptime Monitoring + Server Monitoring + Status Page + Blacklist Monitoring!