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.
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.
Amazon CloudWatch - Amazon CloudWatch is a monitoring service for AWS cloud resources and the applications you run on AWS.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Better Uptime - We call you when your website goes down
DigitalOcean Monitoring - Collect metrics, monitor Droplet performance, and receive alerts when problems arise in your infrastructure—no configuration required.