Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.

Zenmap - Zenmap is the official cross-platform GUI for the Nmap Security Scanner.

Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use

Nmap Online - Nmap Port scanner, OS Detection, Traceroute online

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.

Fing - Discover which devices are connected to any Wi-Fi network, map devices, detect intruders, assess...