UptimeRobot - Free Website Uptime Monitoring
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.
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.
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.
Uptime Doctor - You can monitor up to 5 URLs with 1-minute monitoring interval, for FREE!
Service Uptime - Free website monitoring service checks your site from worldwide locations and alerts you instantly via email or SMS when it becomes unavailable. Detailed uptime statistics and performance reports available.