Joe Maller twiteers about EaKiu, a supercool WiFi spectrum analizer for Mac OS X:

EaKiu offers a number of views of the wireless spectrum including LINEAR, 3D Spectrum, and 3D Interactive graphs. Recording is supported as well as playback of MetaGeek style CSV file formats.

It requires a Wi-Spy USB dongle, which is apparently a software defined radio for the 2.4 Ghz range.