Zabbix is an open-source monitoring software, like Paessler PRTG, which can monitor SQL Server performance statistics and status. Although it doesn’t replace DBA functions, it is useful to provide evidence when things start to fail. Among its features are:
Alert when the service fails or stops. Monitor Windows Performance Counters. CPU usage. Disk latency. Active connections. Blocking queue length. Transactions and resource usage. Dashboards for historical graphic visualization.
By default, there are multiple checks, but it’s totally augmentable so you can create your own monitoring at the OS or database level, integrate other tools’ monitoring into a dashboard