Qualys - Qualys helps your business automate the full spectrum of auditing, compliance and protection of your IT systems and web applications.
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.
OpsDash - All-in-one solution for monitoring servers, services, uptime and databases. Curated Dashboards. Rule-based alerting.
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.
Monitornator - Server monitoring made simple