Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
TamoGraph Site Survey - Wi-Fi site coverage survey and modelling application
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
Waircut - Free network audit app
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.
Ekahau Site Survey - Ekahau Site Survey is a flexible wireless site survey software tool that performs automatic wireless coverage calculations and collects detailed information about your wireless network environment.