Blue Cat
Blue Cat's FreqAnalyst is a free spectrum analyzer plugin that lets you monitor the spectral content of your audio signal in real time. It has been designed to provide extreme smoothness and high resolution for both time and frequency: unlike most spectrum analyzers, Blue Cat's FreqAnalyst is able to display continuous variations even with a very high frequency resolution, thanks to its unique smooth interpolation algorithms.
Most aspects of the audio to frequency display conversion can be managed, which lets you total control over the monitoring. A special thresholding system has been developed to help you see the important parts of the spectrum easily. In case you need extra precision in a particular frequency range, you can zoom the display and check the part of the graph you are interested in.
As shown in the screenshots, you can use the new opacity management feature to make the plug-in window transparent and see what is behind: forget about your multiple display setup, you do not need it anymore! You can now check the spectrum of your mix and see your audio tracks behind. This feature is also particularly useful when you want to compare the spectrum of two tracks: just stack your two FreqAnalyst windows and see the difference!