STSideComp is a free compressor VST plugin developed by STSoft. Compatible OS(s): macOS.
STSideComp is a straightforward, fully parameterized, compressor with a dedicated sidechain input stereo channel. It is designed as a VST effect for Mac OS X and is specially created for and tested with Ableton Live to create typical French House style ducking effects.
http://www.senortorpedo.de/stsoft/