Arctican Audio tells us that The Mid-Sider is a plugin for decoding mid-side type stereo recordings. They say that you simply drop this over a stereo bus with your mid and side channels. By default, the plugin presumes you have your mid channel panned fully left, your side channel panned fully right, and you had the side microphone's front (or positive) side facing left during recording.
The plug-in is Windows VST 32/64-bit at the moment, but OSX support is coming soon
Pricing and Availability:
Massive sound library covers many musical genres