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.
Amazon CloudWatch - Amazon CloudWatch is a monitoring service for AWS cloud resources and the applications you run on AWS.
Google StackDriver - Stackdriver provides monitoring services for cloud-powered applications.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
AWS Config - Cloud Monitoring
IsDown App - Monitor all your cloud services status pages in one place