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).
SiteUptime - Siteuptime provides website monitoring services. Receive instant email and SMS alerts when your website becomes unavailable. View detailed uptime statistics and performance reports for your website monitors.
Site monitoring - Free open-source website monitoring software, easy to set up and use for monitoring web sites.