Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
inSSIDer - inSSIDer Essential makes it easy to maintain small business WiFi.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
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.
Homedale - With Homedale you can search for Wi-Fi / WLAN Access Points and monitor their signal strength.