From AZO Technologies:
VisiWave Site Survey SO is a complete 802.11a/b/g site surveying solution. You collect survey data using a standard wireless adapter while walking around the survey area clicking on a floor plan image or using a GPS receiver to track your location. The survey data is mapped using the included reporting application which provides rich and easily comprehensible views of the complex and voluminous data captured during a site survey. This allows you to quickly assess your overall Wi-Fi coverage and to drill into great detail on specific areas of interest.
Version 2.0.8 includes Wi-Spy 2.4i support, create very large Google Earth reports, get notified when the survey file changes, highlight interference regions in Google Earth reports.