As the 2.4 GHz ISM Band becomes more popular (and more congested) interference from non-Wifi devices can seriously degrade Wi-Fi performance.

Wi-Spy is the world's smallest 2.4 GHz spectrum analyzer, and has been designed specifically for troubleshooting and analyzing Wi-Fi networks. It shows the signal strength for all Wi-Fi networks as well as Bluetooth, 2.4 GHz cordless phones, microwaves, Zigbee, and all other 2.4 GHz transmitters. At a glance you can see what Wi-Fi channels are quiet and what channels you should avoid if possible. This makes it easy to pick a quiet channel for your access point instead of picking your favorite number between 1 and 11.