A1StereoControl

With my A1StereoControl plugin you will be able to expand or limit the STEREO WIDTH of your tracks using only one single knob. This powerful technique can be used on single tracks or groups tracks while mixing or even on a master bus in final mastering situations.
As many experienced mixing & mastering engeneers worldwide are complaining, stereo-expansion can have unwanted side-effects. Especially the bass frequencies should normally be simple mono.
As you may guess, my plugin has also a solution for this problem. The integrated "Safe Bass" algorithm centers all bass frequencies below a adjustable value right back into the middle of the stereo field. This gives your tracks the solidity and definition you want while preserving maximum transpancy and sharpness. Similar techniques were used by professional studio producers over ages
with either expansive hardware devices like elliptical equalizers or even advanced m/s-processors. With my plugin you will be able to manage this complicated task with a minimum effort.

http://www.alexhilton.net/A1AUDIO/index.php/a1stereocontrol

DMGaudio TrackControl

TrackControl is an all-in-one channel strip plugin featuring trim and gain controls, channel swapping and MS/LR conversion, phase inversion (per channel), stereo width, pan and balance controls, channel delay (both backward and forward) and a high-pass filter. The output control (located in the menu bar) allows the user to monitor the Mid/Side and Left/Right components of the processed audio signal individually. A patch manager is also included, along with a bypass switch and input/output gain monitoring. It’s also worth pointing out that the plugin will only display a limited amount of controls when loaded on a mono channel, to avoid any confusion (stereo balance, pan, and width controls are left out in this case).

https://dmgaudio.com/trackcontrol

Bob Perry Listener

Bob Perry Listener is the ultimate tool for checking your stereo signals.

Monitor your left and right signals individually, listen to side and mid signals with the integrated M/S matrix.

• Listen to mid signal (on both output channels)
• Listen to side signal (on both output channels)
• Swap left and right signals
• Mute left and/or right signals
• Phase invert left and/or right signals
• Monitor output levels

https://www.bobperry-audio.com/shop/bob-perry-listener/

Flux bittersweet

Bittersweet v3 is a free software processing tool for managing audio transients. Turning the big wheel button on the sweet side, decreases the transients amplitude. On the bitter side the transients amplitude is magnified.

Also: Stereo Tool v3 features ultra precise control of input gain and individual pan for left and right channels. A phase inverter is available on each channel. Global stereo pan and stereo width settings are also implemented to complete the management of the stereo signal.
Stereo Tool v3 also offers accurate visual feedback reflecting the signal content. A vector scope display, PPM meters for both inputs and outputs, and a phase correlation meter permanently monitor the signal.
http://www.fluxhome.com/products/freewares/bittersweet-v3