Cloudflare DNS - Install the free app that makes your phone’s Internet more fast, private, and reliable.
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
GoDaddy Premium DNS - Sell your domain name in the Premium Domain Names section of the domain search page of the world's largest Domain Name Registrar - GoDaddy.
Better Uptime - We call you when your website goes down