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
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.
Waircut - Free network audit app
Wireless Network Watcher - scans your wireless network and displays the list of all computers and devices that are currently connected to your network.
Homedale - With Homedale you can search for Wi-Fi / WLAN Access Points and monitor their signal strength.