TamoGraph Site Survey - Wi-Fi site coverage survey and modelling application
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Waircut - Free network audit app
Fing - Discover which devices are connected to any Wi-Fi network, map devices, detect intruders, assess...
Homedale - With Homedale you can search for Wi-Fi / WLAN Access Points and monitor their signal strength.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use