inSSIDer - inSSIDer Essential makes it easy to maintain small business WiFi.
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
Wifi Analyzer - Wifi Analyzer is a handy tool which helps you to select a better channel for your wireless router.
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.