UptimeRobot - Free Website Uptime Monitoring
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
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.
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
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.