HeatMapper - Free Wi-Fi Coverage Mapping Tool

http://www.ekahau.com/products/heatmapper/overview.html

Ekahau HeatMapper is a free software tool for quick and easy coverage mapping of Wi-Fi (802.11) networks. It's the only free, easy-to-use tool that shows, on a map, the wireless network coverage in your home or small office. HeatMapper also locates all access points.