Qualys - Qualys helps your business automate the full spectrum of auditing, compliance and protection of your IT systems and web applications.
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.
AlienVault USM (from AT&T Cybersecurity) - AlienVault USM Anywhere delivers powerful threat detection, incident response, and compliance management for cloud, on-premises, and hybrid environments.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
AWS Artifact - Cloud Compliance
Better Uptime - We call you when your website goes down