ATKSideChainExpander is a free VST plugin. This stereo expander can work on two channels, left/right or middle/side, possibly in linked mode (only one set of parameters), and can be set up to mix the input signal with the compressed signal (serial/parallel compression).
The side chain channels can be used to steer the gain stage (the same setup will be used, right/left or middle/side).
The supported formats are:
VST2 (32bits/64bits on Windows, 64bits on OS X)
VST3 (32bits/64bits on Windows, 64bits on OS X)
Audio Unit (64bits, OS X)