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.).
Datadog offers a free trial plan for 14 days to monitor as many servers as you like; no credit card is required.
Zabbix is an open-source monitoring software tool for various IT components, including networks, servers, applications, databases, virtual machines, and cloud services.
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.