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
Zerigo DNS - Zerigo is a cloud infrastructure provider, offering enterprise cloud servers, VMware View DaaS, VPS, and cloud-enabled DNS with built-in server monitoring.
Better Uptime - We call you when your website goes down
Google Cloud DNS - Reliable, resilient, low-latency DNS serving from Google’s worldwide network of Anycast DNS servers.