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.
Dnsstuff - DNSstuff offers DNS tools, Network tools, Email tools, DNS reporting and IP information gathering.
DownDetector - A simple service to tell you whether a website is down.
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.