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.
HostedScan.com - Online vulnerability scanner for servers, networks, and web applications.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
HackerTarget.com - Security Vulnerability Scanning based on Open Source Tools.
Better Uptime - We call you when your website goes down
Intruder - Intruder is a security monitoring platform for internet-facing systems.