Amazon CloudWatch - Amazon CloudWatch is a monitoring service for AWS cloud resources and the applications you run on AWS.
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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
DigitalOcean Monitoring - Collect metrics, monitor Droplet performance, and receive alerts when problems arise in your infrastructure—no configuration required.
IsDown App - Monitor all your cloud services with our status page aggregator. IT teams choose IsDown to monitor all their vendors and get alerts whenever an outage occurs. Integration with Datadog, PagerDuty, Slack, Microsoft Teams, and a lot more.
Google StackDriver - Stackdriver provides monitoring services for cloud-powered applications.