NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
Nessus - Nessus Professional is a security platform designed for businesses who want to protect the security of themselves, their clients, and their customers.
SolarWinds System Management - Performance monitoring and management for applications, Websites, servers, storage arrays and virtual infrastructure.
Tenable.io - Tenable.io Cyber Exposure platform helps to protect any asset on any computing platform and eliminate blind spots.
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.