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!
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.
Site monitoring - Free open-source website monitoring software, easy to set up and use for monitoring web sites.
UptimeRobot - Free Website Uptime Monitoring
Happy Apps - Uptime Monitoring Simplified | Happy Apps
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.