From: http://www.metageek.net/

  Wi-Spy is a custom USB spectrum analyzer designed specifically for troubleshooting Wi-Fi issues. 
  Unlike most Wi-Fi tools, Wi-Spy is able to measure ALL RF signals, not just Wi-Fi traffic. With 
  Wi-Spy DBx, you can measure RF signals in the 5 GHz band, which is quickly becoming as crowded 
  as the 2.4 GHz band!