Metageek has a nice product, Wi Spy, a combination of an USB dongle and a Windows Software that let you analyze frequencies around 2.4 Ghz :
Wi-Spy™ is the world’s smallest 2.4 GHz spectrum analyzer*. Wi-Spy is perfect for troubleshooting interference from the following devices:
Wi-Fi (802.11 b/g/n)
Microwave Ovens
Cordless Phones
Zigbee
Bluetooth
Wi-Spy is priced at $99. It’s been reviewed Tom’s networking here.
Some open source tools are available for Linux and OSX supporting the Wi-Spy hardware:
Wispy-Raw – A basic dumper from the USB interface to stdout, without RSSI to dBm conversion.
Wispy-Curses – A simple libcurses text-based graphing tool.
Wispy-GTK – A full GTK grapher, similar to the windows graphing tool

