Matthieu Brucher has released version 1.0.1 of ATKSideChainExpander, a free side-chain stereo expander effect plug-in based on the Audio Toolkit.
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).
ATKSideChainExpander is available for Windows and Mac (VST/AU). Donations are appreciated.