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.
UptimeRobot - Free Website Uptime Monitoring
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.
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.
Uptime Doctor - You can monitor up to 5 URLs with 1-minute monitoring interval, for FREE!