StatusNotify - StatusNotify monitors the status of 250+ cloud services.
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.
IsDown App - Monitor all your cloud services status pages in one place
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
DownDetector - A simple service to tell you whether a website is down.
Better Uptime - We call you when your website goes down