Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
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.
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.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
Nagios - Complete monitoring and alerting for servers, switches, applications, and services