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:

Here you'll find the plugins of the HOFA 4U collection directly out of the HOFA-studios. And as it is common for freeware, all HOFA 4U plugins are free and up for grabs for everyone. VST 4 FREE - Free Audio Plug-ins and Archives Free audio plugins archive - Instruments and effects for audio software. Plugins for Cubase, FL studio, Reaper, Ableton and other VST/AU platforms. A VST plugin containing a Correlation Meter and a Goniometer (Vector Scope) that will help you get a reliable indication for mono compatibility. Vintage Simply String 2 is 100% free Sample-based multi platform synthesizer plugins from RDGAudio. It comes with VST and AU formats. Loads very fast, easy to use, ADSR and built-in.

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.

Gonio by UKM is a Virtual Effect Audio Plugin for Windows. It functions as a VST Plugin. Free VST Plugins are one of the most exciting things about mixing. In the past it was impossible to imagine that so many quality tools could be available at no cost. Today there are more great free plugins than ever before, but the freeware scene is always changing. A robust pedalboard simulator, Guitar Rig 5 player is one of the best free VST plugins that comes packed with a Shoegazer’s dream effects: heavy reverb, delay, phaser, chorus, flanger, you name it.

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/

Goniometer Vst Plugin Freeware Plugins

1.5) MultiInspector free

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

2) Sonogram

2.1) ag-works sonogram sg-1 Best pro vst plugins.

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 ToolDistortion vst plugins ableton.

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 :

Dust Analyzer:

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

Goniometer Vst Plugin Freeware Software

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