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.
UptimeRobot - Free Website Uptime Monitoring
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
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.
Site monitoring - Free open-source website monitoring software, easy to set up and use for monitoring web sites.