NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Kismet - Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
Wifiner - WiFi Analyzer - WiFi Analyzer. See your WiFi signal on a visually comprehensive map and make sure you are getting the best of your network
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.