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.
Wireless Network Watcher - scans your wireless network and displays the list of all computers and devices that are currently connected to your network.
Prometheus - An open-source systems monitoring and alerting toolkit.
Advanced IP Scanner - Advanced IP Scanner shows all network devices, gives you access to shared folders, and can even remotely switch computers off. Download it Free.