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.
WP Umbrella - The easiest way to manage multiple WordPress websites effortlessly.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Datafold - Quality assurance & monitoring for analytical data
Better Uptime - We call you when your website goes down
Orbitype - SEO & workflow focused WordPress admin