If you use Wavosaur for mastering / premastering, or signal analysis in a more general way, you need tools for audio analysis : spectrum analyzers, phase analyzers, sonogram.
Wavosaur comes with analyzers, you’ll find them in the ‘tools’ menu, along with oscilloscopes & statistics about the sound file (maximum sample peak, RMS power, average value..).

But you can also use VST plugins (for some people it can be more convenient).
We have searched deep deep inside deep deep down inside into the web, to find the best free VST plugins for audio analysis, here’s the selection:

SpectrumAnalyzer is a state-of-the-art spectrum analyzer plugin, which offers a lot of opportunities to display the spectral content of your audio material and helps you create a better mix this way. SpectrumAnalyzer comes in two editions, a free and a full version.

1) Spectrum Analysis

1.1) Voxengo SPAN

This spectrum analyzer uses FFT to display the signal frequency in real time.
You can freeze the display, and use the mouse to shows dB values / Frequency value
Frequency is displayed in Hz and in musical note.
The Voxengo SPAN analyzer also displays statistics : RMS power, and it features a clipping detection.

More info and download @ http://www.voxengo.com/product/span/
It is worth mentioning these plugins too, freeware!

1.2) Mdsp SmartElectronix Fre(a)koscope

Frequency Analyzer Vst Download Free

Very nice, with interesting options for the spectrum display : linear/logarithmic/semitones ..
Adjustable size for the FFT buffer. Had sometimes display problem with Wavosaur.
More info and download @ http://mdsp.smartelectronix.com/freakoscope/

1.3) BlueCat FreqAnalyst

Like the previous VST effect, this one can show spectrum of left / right channel or both, has a “hold peak” feature that is interesting, when dealing with EQ level in your mix.
More info & download @ http://www.bluecataudio.com/Products/Product_FreqAnalyst/

1.4) Seven phase Spectrum Analyzer

This one is freely resizable inside the host application (works in Wavosaur)
More info and download @ http://sevenphases.wordpress.com/spectrum-analyzer/

1.5) MultiInspector free

Free Spectrum Analyzer Vst

Download @ http://www.vertexdsp.com/downloads.html#MultiInspectorFree

2) Sonogram

Frequency Analyzer Vst Download Full

2.1) ag-works sonogram sg-1

This free plugin displays a scrolling sonogram for frequency analysis.
You have the choice for the sonogram colors, and many options to tweak the sonogram displays
This is the only sonogram VST we have found for free ! Fortunaly it’s very good.
More info & download @ http://ag-works.net/plugins.sg1.htm

3) Goniometer

3.1) Flux Stereo Tool

When you need to visualize your stereo position, and the repartition in the field, this plugin is very useful. You can check for phase errors in your mixes.
download @ https://www.fluxhome.com/download

3.2) Gonio3

download @ http://www.uk-music.de/index.php?page=downloads

3.3) HOFA 4U Goniometer & Korrelator

download @ http://hofa-plugins.de/pages/start_en/4u_en.php

If you think that we have forget some free VST analyzer plugin in this list, do not hesitate to comment !

Edit:

Frequency Analyzer Vst Download For Windows 10

We just have found this pretty nice analyzer somewhere on our harddisk :

Dust Analyzer:

It provides three modes : spectrum / scope / phase , with “hold peak” option, freeze option, dB/Freq display with mouse, linear / logarithmic scale

Frequency Analyzer Vst Download Free

more info & download @ KvR audio forum & http://www.signaldust.com/