Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
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.
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
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.