Qualys - Qualys helps your business automate the full spectrum of auditing, compliance and protection of your IT systems and web applications.
UptimeRobot - Free Website Uptime Monitoring
Burp Suite - Burp Suite is an integrated platform for performing security testing of 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.
OpenVAS - The Open Vulnerability Assessment System (OpenVAS) is a framework of several services and tools...
Uptime Kuma - A fancy self-hosted monitoring tool.