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.
Sucuri - Website Protection, Malware Removal, and Blacklist Prevention
UptimeRobot - Free Website Uptime Monitoring
Amazon CloudFront - Amazon CloudFront is a content delivery web service.
SiteUptime - Siteuptime provides website monitoring services. Receive instant email and SMS alerts when your website becomes unavailable. View detailed uptime statistics and performance reports for your website monitors.
CloudFlare - Cloudflare is a global network designed to make everything you connect to the Internet secure, private, fast, and reliable.