DNS Made Easy - DNS performance, reliability, and security have never been easier.
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 Route 53 - Amazon Route 53 is a highly available and scalable DNS web service.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Akamai Fast DNS - Akamai provides cloud services for helping enterprises provide secure, high-performing user experiences on any device, anywhere.
Better Uptime - We call you when your website goes down