Nagios is an open-source network monitoring for Linux, Windows, and Unix that help monitor network for problems caused by overloaded data links or network connections and monitoring routers, switches, and more.
Nagios comes with a fully-featured 30-day trial and is free for up to 7 nodes after the trial.
Zabbix is an open-source monitoring software tool for various IT components, including networks, servers, applications, databases, virtual machines, and cloud services.
Datadog is a monitoring and analytics tool that focus on infrastructure monitoring and cloud services that cover all relevant infrastructure monitoring parameters (metrics, logs, security, etc.).