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.
ViewDNS.info - Free DNS related tools including Reverse IP Lookup, DNS Propagation Checker, Chinese Firewall Test...
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Whats My DNS - Instantly check your domain names current IP address and DNS record information against multiple name servers located in different parts of the world.
Better Uptime - We call you when your website goes down
NsLookup.io - A simple and powerful tool for querying DNS records