Wi-Spy is perhaps 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, which makes it easy to diagnose and select the optimum wireless channel for your network or devices.

Wi-Spy is available from ThinkGeek.com  for US$ 99.99 and it works with Windows XP, 2000, Mac and Linux.  A must have if you are a network administrator or just a wi-fi spy ;)