OnlineOrNot - Uptime monitoring for software teams (even teams of one).
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
UptimeRobot - Free Website Uptime Monitoring
Uptime Kuma - A fancy self-hosted monitoring tool.
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
WebmasterSidekick - It is an All-In-One Site Monitoring & SEO Tool that Reports site Performance, Monitoring Key...