Dnsstuff - DNSstuff offers DNS tools, Network tools, Email tools, DNS reporting and IP information gathering.
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.
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
SolveDNS - SolveDNS works to check your name servers, a records, and mail servers, SOA records for errors, consistency, efficiency and speed using several DNS tests.
Better Uptime - We call you when your website goes down