We are now in the download and voting stage of the KVR Developer Challenge 2012. This news item features an entry called Spectrum Analyzer S8 by Kevin Kripper. You can download it for free here (Windows & Mac OS X).
Here's what they say about it:
Spectrum Analyzer S8 is a free multiple-track real time spectrum analyzer. It lets you visualize with different colors the spectral content of 8 stereo tracks simultaneously.
You can use it as a frequency overlap detector and actually see which part of the spectrum every single instrument uses. The ideal solution for mixing!