Amazon Route 53 - Amazon Route 53 is a highly available and scalable DNS web service.
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.
DNS Made Easy - DNS performance, reliability, and security have never been easier.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
GoDaddy DNS - GoDaddy DNS helps manage website performance, reliability and security.
Better Uptime - We call you when your website goes down