reFuse Software has released version 0.51 of Mid-Side Suite, which consists of two plug-ins: MS from LR (the encoder) and MS to LR (the decoder).

There are two primary uses for these:

1. You have recorded material using a mid-side microphone pair, and you want to decode this on playback into stereo. In this case, you would simply place the MS to LR plug-in on a stereo buss, and feed the L side of the buss the mid mic, and the R side of the buss the side mic. Use the stereo width knob on the MS to LR to determine the stereo spread.
2. Alternately, you can use both these plug-ins to selectively process mid and side information from a standard stereo track. In this case, you would place MS from LR at the start of your effects chain. Then, insert other plug-ins, like compression or EQ, and set these to operate in multi-mono mode. This will give you independent control over the two channels. You can now, for example, apply compression to just the mid channel, or place a hi-pass filter on the side channel... or whatever other crazy stuff you wanna try. At the end of the effects chain, place a MS to LR plug-in, to bring the two channels back to LR stereo. If you don't want to affect the stereo spread, just leave the dial at 50/50, which is the default setting.

Mid-Side Suite is free and is available for Mac OS X. It uses the Pluggo standard, and requires installation of the free Pluggo Jr or Pluggo Runtime in order to function.