Retracing the trend of free plugins, here is a series of goodies to be included in your home mastering chains, accompanied by a brief illustration of the main controls.
TDR FEEDBACK COMPRESSOR II
The compressor in question is very particular because it uses a feedback type circuit and has some parameters that most dynamics processors do not have; in general it has a transparent response with a slight introduction of harmonic distortion (be careful not to overdo the ratio).
Let's see some of the parameters on the interface:

I think it is an excellent plugin, on a par with famous colleagues, more or less cheap; it is not easy to manage due to the presence of different controls, but at the same time, it allows to operate in detail on a wide range of signals and in different situations. Having become aware and mastering all virtual rotators, good results can be obtained.
Also worth trying on individual instruments such as kick drum, snare drum and guitar.
USEFUL INFORMATION
Download: www.tokyodawn.net/tdr-feedback-compressor-2
(windows / mac VST / AU)
BAXTER EQ

The second processor you see in the figure above, belonging to the same developer as the previous plugin, is a Eq with shelving filter. This type of equalizers allows you to have control of certain areas of the audible frequencies.
Unlike low-pass and high-pass filters, the terms high and low refer to the frequencies on which we want to act by modifying their amplitude, therefore with the term cut we indicate an attenuation and with the term boost an emphasis of the same.
In general this plugin is very suitable for the mastering chain, the filters are accurate in the response and above all very transparent on the low frequencies. The mid-side encoding gives excellent results on acoustic or very “rock” tracks, but other possibilities must not be excluded… experimented!
USEFUL INFORMATION
download: http://varietyofsound.wordpress.com/downloads
(win x32 VSTs)
FERRIC TDS - Tape Dynamics Simulator

As you can guess from the name, this plugin simulates the sound effects (the nice ones) of tape recorders, so we can add harmonic content with saturation effects, get a less edgy dynamic response and control the output audio peaks.
The knob recovery regulates the speed at which the processor starts spinning again after a work cycle (it could be seen as a relative of the parameter attack of a compressor).
So by squeezing too much the dynamics of a snare drum with dynamics you can compensate for the evident compression effect and the loss of transients, slowing down the recovery time with recovery. Furthermore, it must be considered that the faster the recovery, the more you run the risk of introducing distortion in the signal (as reported by the developer, among other things).
The mode class responds to the typical saturation of a tape recorder by attenuating the low frequencies, emphasizing the mid frequencies by rounding the high frequencies a little, the modern mode responds better to the low frequencies and has better control over the peaks. Based on your needs, choose which of the two types is right for you.
When using the saturation control it is necessary to pay attention to the distortion introduced and remember that the best use of a processor is to improve a sound without making it perceive its presence.
USEFUL INFORMATION
Download: http://varietyofsound.wordpress.com/downloads
(win x32 VSTs)
STEREO TOOL

This simple, but no less important plugin produced by FLUX, if inserted among the first elements of the mastering effects chain, can be very useful. It can control the input signal individually on the left and right channel, it can invert the phase of the single channel, widen or narrow the stereo image by varying the balance between the central sounds and those placed on the sides (acting on width), measure phase correlation and monocompatibility.
USEFUL INFORMATION
Download: www.fluxhome.com/download
(windows, mac, VST / AU / RTAS)
FLUX - BITTER SWEET II

Download: www.fluxhome.com
Among the FLUX products we can also introduce this transient designer that allows you to emphasize or attenuate certain sounds in a mix by acting on the transients of the input signal.
Il transient designer it mainly affects the attack and sustain phase of a sound. Unlike a compressor, it does not have a settable threshold, so it will act in the same way on transients regardless of their input level; it can be very useful both in mastering and mixing on drum tracks, percussion, guitars, bass or vocals, to give a little punch to the sound, to soften the attack, to correct errors in the recovery phase. Having few parameters to set, the Bitter Sweet II is simple, immediate and creative to use. Depending on the situation it can be inserted into the effects chain together with other dynamics processors or in their place.
Finally, turning the central knob to the right will increase the peak level, so watch out for the signal output!
LIMITER N ° 6

This plugin is a very complex limiter. On the developer's website you can find a short video tutorial http://vladgsound.wordpress.com/plugins/limiter6 .
Given the presence of numerous parameters, we will only introduce, in an overview, the main functions:
This limiter is of excellent workmanship, designed so that the various sections, set at their best, can give precise and natural results; for those wishing to use only one or more modules, there is the possibility to deactivate them individually with the switch on off.
For those who do not have major problems with English, the user manual (well done too), will clarify any doubts.
USEFUL INFORMATION
Download: http://vladgsound.wordpress.com/downloads
(windows, mac VST / AU)
Finally, here is an audio and visual example of how to use all the plugins presented in a mastering effects chain. In some cases I have made the processor effect more noticeable by exaggerating a bit with the controls.
Experience the many possibilities offered by combining these plugins or by integrating them with those you usually use.
Good experimentation to all
Original track
Trace processed