Our experience is that e-mail is a huge time-thief. That is why we migrate as many services and functions from e-mail to Slack.com as we can.
The following four free services supports Slack.com:
Fake warnings about service down
CloudFlare.com will give fake warnings unless you whitelist the service in the CloudFlare firewall.
Lists of IP addresses
UptimeRobot IP to be whitelisted in CloudFlare.com firewall
Check some tekst in the robots.txt file
You can use some text in your robots.txt file for verifying that your site is running.