Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
SEMRush - All-in-one Marketing Toolkit for digital marketing professionals.
UptimeRobot - Free Website Uptime Monitoring
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.
Simple Ops - Simplify website performance and uptime monitoring with alerting, ssl check, chrome ux metrics, multi locations
Moz - Backed by industry-leading data and the largest community of SEOs on the planet, Moz builds tools that make inbound marketing easy.