NameserverChecker.com - Nameserver Checker Tool
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.
CheckNameserver.com - Nameserver Checker Tools
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
NameserverFinder.com - Nameserver Lookup Tool
Better Uptime - We call you when your website goes down