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.
WebmasterSidekick - It is an All-In-One Site Monitoring & SEO Tool that Reports site Performance, Monitoring Key...
Site monitoring - Free open-source website monitoring software, easy to set up and use for monitoring web sites.
Uptime Kuma - A fancy self-hosted monitoring tool.
UpTower - UpTower is an easy-use tool for website monitoring. UpTower checks websites in 1-minute intervals and notifies user via push notifications about every outage immediately. Using UpTower app user can monitor websites and their availability for free.