Arduino forum user [Blibo] shares its 2.4 Ghz spectrum analyzer project on the forum. The project is based on the CYWM6935 board (wireless), an Atmega 328 and a Nokia 5110 LCD-

I finished the (mostly) permanent version of my 2.4ghz spectrum analyzer, and soldered it up. I included 3 modes for scanning (fast, slow, and ghost – like the long exposure on a camera), plus a function to display the voltage on an analog pin, and graph it (for when the oscilloscope’s not cooperating). These modes are toggled through by hitting the big push button […] I have already used it to help setup my wireless network, (channel, location, things that cause interference), and it is always interesting to see what uses the 2.4ghz spectrum. So far, the things that I’ve noticed on the spectrum while walking around with the analyzer are: 
-microwave ovens (huge disturbance in the middle of the spectrum)
-Wifi 
-Cordless phones
-Bluetooth
-Wireless keyboard
-Wireless speakers
The fast mode is ok for seeing EMI, but for digital signals, the slow mode is best. The ghost mode also gives a general idea of spectrum use over a period of time.

via [HackADay] source(code) on [Arduino Forum]