http://mdsp.smartelectronix.com/mstools/
These VST effect plugins can be used to process the Mid and Sides of audio separately. At the very least, msEncode and msDecode can be used with the MTS StereoEffect plugin for your own processing tricks of Mid and Sides.
For example, load a MultiEffect plugin into a slot. Then load msEncode into the first slot of the MultiEffect and msDecode into it's last slot. Next load a StereoEffect plugin into a slot in between. Now when you load effects into the StereoEffect plugin, instead of processing Left and Right, it will process Mid and Sides. As a bonus, the StereoEffect plugin's volume controls can be used as balance controls between Mid and Sides.
I just started doing experimenting with this, doing different things to one channel than the other can really result in some interesting changes in sound quality.
Have fun