You can use SoundFlower to redirect audio out from any application to any other application. This removes the need to find an application that can 'snoop' the audio stream of any other application. Instead, you just route audio out from App A to SoundFlower, and then audio from from App B from SoundFlower and the two are connected.
With SoundFlower installed you could route audio from an app in to Logic and use it's spectrum analyzer tool.
If Logic is too expensive for you, you can look at combining something like Audacity (free) with Voxengo's SPAN which is a free, FFT-type spectrum analysis VST.
I'm most familiar with the plugin + DAW route because that's what I know. But for stand-alone spectrum analyzer offerings you can check out some of these:
Hit Squad has a list of plugins and standalones for signal analysis on the Mac. Those two stood out when I scanned through it for non-plugin options.