Contrary to other stereo recording techniques (X/Y, Bloomlein), M/S recordings need to be decoded to stereo before they are useful. The decoding technique is relatively easy to implement in any DAW or audio editor. There are even specialised plugins that will do this for you.
But, if you speak Bash, do read on!
For a geek like me, opening a DAW or plugin just to decode a bunch of audio files is overkill. Don't care about GUIs either. So, I coded a simple command line tool to decode my MS audio files.
The result is msproc. It is a GPLv2 command line utility made with JUCE that decodes MS recordings to stereo. Right now it's tested only on OSX. Should be straightforward to compile for other platforms though. If you are an OSX user, you can take a precompiled msproc.v1.0.0 for a spin.
Feel free to request features or report bugs here