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.
Google StackDriver - Stackdriver provides monitoring services for cloud-powered applications.
MongoDB Cloud Manager - Cloud Manager saves you time, money, and helps you protect your customer experience by eliminating the guesswork from running MongoDB.
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.