NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.

Qualys - Qualys helps your business automate the full spectrum of auditing, compliance and protection of your IT systems and web applications.

Server Monitoring - A server monitor is a tool that monitors your server and website on close intervals and reports to you if it's inaccessible.

Nessus - Nessus Professional is a security platform designed for businesses who want to protect the security of themselves, their clients, and their customers.

Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.

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.