Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
SEMRush - All-in-one Marketing Toolkit for digital marketing professionals.
UptimeRobot - Free Website Uptime Monitoring
Screaming Frog SEO Spider - The Screaming Frog SEO Spider is a small desktop program (PC or Mac) which crawls websites’ links...
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Pulno - SEO Audit and Website Analysis. Pull up your SEO.