Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
intoDNS - IntoDNS checks the health and configuration and provides DNS report and mail servers report.
Pingbreak - Pingbreak is an original and free monitoring service.
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
DownDetector - A simple service to tell you whether a website is down.
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.