Website Checker Tech - WebsiteChecker.Tech is SaaS tool that detects SEO and availability issues of your website, making it easier for you to fix them.
UptimeRobot - Free Website Uptime Monitoring
SEMRush - All-in-one Marketing Toolkit for digital marketing professionals.
Uptime Kuma - A fancy self-hosted monitoring tool.
Sitechecker - All-in-one SEO platform including rank tracker, website SEO audit tool and website changes monitoring. Detailed reports by 100+ parameters with how-to-fix guides.
OnlineOrNot - Uptime monitoring for software teams (even teams of one).