What's the point of the plugin? They just produce stereo L and R from M and S, which you can do very simply by the copy/invert/pan method. This gives you of course three potential mixer channels that can have processing added, but you just need to experiment to discover which effects work, and which wreck the stereo decoding. In general the side channel is a bit hollow and reverberant if the recording space was live, but the benefit of M/S is simply being able to close down, or expand the width by balancing the amount of both side channels against the centre channel. Small adjustments can be very startling, assuming your monitor system is good. If your monitoring is on headphones or speakers that have that 'hole in the middle' problem, then your M/S mixes will be very poor.
Voxengo MSED, as mentioned above. Stands for Mid Side Encode Decode and can do be used to do either or both (encode and/or decode). By itself it can be used to change the relative balance of mid to side. As an encoder it changes L/R to M/S. As a decoder it puts it back to L/R. I wouldn't have it if it wasn't free. This is extremely simple math - doesnt do anything your DAW faders don't do by themselves - and therefore just really can't be done by one company any better than another.
