Site monitoring - Free open-source website monitoring software, easy to set up and use for monitoring web sites.
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!
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
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.
Happy Apps - Uptime Monitoring Simplified | Happy Apps
OnlineOrNot - Uptime monitoring for software teams (even teams of one).