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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Uptime Doctor - You can monitor up to 5 URLs with 1-minute monitoring interval, for FREE!
Cachet - Cachet is a free, open source status page for your API, service or company. Built with all of the features that you'd expect from a status page.