Definitely one of the best free VST plugins you ought to have in your VST rig! 33) MX Project T. Rex 606 Modelled after the vintage Roland TR-606 drum machine of yore, The T.REX 606 is a beast in its own right with some added bells and whistles. Best Free Vocal VST Plugins. It's well-known that dynamic sounds are much harder to mix – and the vocals are super dynamic. Luckily, there is no need to stress out every time you get to the mixer and start working on getting the “perfect vocals.” All you need to do is to know which free vocal mastering VST plugins to use. The IIEQ is a fully parametric 10-band free VST equalizer. The 1st and the 10th band are low and high shelf filters, respectively; all other bands are peak filters. Individual bands can be switched on and off. IIEQ comes with different GUIs of which some are available only in 32-bit.

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:

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

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

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

2) Sonogram

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:

We just have found this pretty nice analyzer somewhere on our harddisk : 3d vst plugin.

Dust Analyzer:

Free Peaking Filter Vst Tool

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

Free Peaking Filter Vst Plugin

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