UptimeRobot - Free Website Uptime Monitoring
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!
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.
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.
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
Site monitoring - Free open-source website monitoring software, easy to set up and use for monitoring web sites.