Dynu Dynamic DNS - Free dynamic DNS service with powerful features
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.
Dyn - Managed DNS, Outsourced DNS & Anycast DNS
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
ClouDNS - ClouDNS is a platform that allows users to keep their websites, data, and network security all the time.
Better Uptime - We call you when your website goes down