StatusNotify - StatusNotify monitors the status of 250+ cloud services.
StatusGator - Monitor all your hosted applications, vendors, and websites, and keep your team informed of outages. StatusGator is the world's premier status aggregator, status page, and website monitoring tool.
IsDown App - Monitor all your cloud services status pages in one place
Amazon CloudWatch - Amazon CloudWatch is a monitoring service for AWS cloud resources and the applications you run on AWS.
DownDetector - A simple service to tell you whether a website is down.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring