[Note before: I don't know why it is described on KVR as "No Longer Available", you can still download it at the Voxengo website for free.].
Might be the best free M/S-Tool on the market.
It does allow you to do so much things when you dig around with M/S-processing.
You can not only use it as M/S-encoder and decoder, it also gives the possibility to adjust, the volume of sum (mid) and the difference (sides), pan them whereever you want in the stereofield, swap 180* degree phase, swap them with each other and mute to listen to one specific channel for mixing. Plus you have a vectoscope to the right side to visualize the changes in real time and a correlator and stereo balance viszualizer at the bottom.
Also as it is a Voxengo plugin, you can costumize the visualizations of the plugin, like I encounter nowhere else on the market.
Really great tool every mixer should have unless you haven't something similar (which will be hard to find).
Candy at last: A TIP you can find in the manual: By clicking CTRL/CMD and dragging either Mid or the SIDE one of the Gain or Pan knobs, the other also moves in parallel to the movement of your mouse. If you do RIGHTMOUSE-click and drag, the movements are interpolated.