KnownOutage - Quickly and easily alert people when there is an issue or outage with your service.
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.
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
Stashboard - Stashboard is a status dashboard for APIs and software services.
Better Uptime - We call you when your website goes down