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.
CloudFlare - Cloudflare is a global network designed to make everything you connect to the Internet secure, private, fast, and reliable.
UptimeRobot - Free Website Uptime Monitoring
Amazon CloudFront - Amazon CloudFront is a content delivery web service.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
KeyCDN - KeyCDN is a high-performance Content Delivery Network (CDN). Lowest price globally at $0.04/GB with HTTP/2 Support and free Origin Shield.