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.
StatusPage.io - StatusPage.io is the best way for web infrastructure, developer API, and SaaS companies to get set up with their very own status page in minutes. Integrate public metrics and allow your customers to subscribe to be updated automatically.
UptimeRobot - Free Website Uptime Monitoring
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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Statping - Status Page for monitoring your websites and applications with beautiful graphs, analytics, and plugins. Run on any type of environment.