Zabbix - Track, record, alert and visualize performance and availability of IT resources
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
Fing - Discover which devices are connected to any Wi-Fi network, map devices, detect intruders, assess...
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
SoftPerfect Network Scanner - Powerful multipurpose network administration tool for Windows and macOS