Opserver
Stack Exchange's Monitoring System
...It functions as a centralized dashboard capable of independently monitoring servers, databases, caches, and network components while aggregating them into a unified operational view. The platform supports a wide range of integrations including SQL Server, Redis, Elasticsearch, HAProxy, and external monitoring systems, making it highly extensible. Built on .NET technologies, Opserver can run across multiple operating systems and is designed for environments that require both infrastructure metrics and application-level diagnostics. It emphasizes real-time status visualization combined with drill-down capabilities for troubleshooting complex distributed systems.