If you’re looking for sidechaining plugin effects the compressor plugin from Slim Slow Slider is an excellent addition to your VST plugin folder.
The Sidechain compressor comes in a stereo and a mono version.
Really quite easy to use, just add the effect to the effects bin of the track you want to affect. Then route the track you want to ‘influence’ the compressor via it’s output. Click here for Sonar Sidechain tutorial, principle should work for other DAWs.
Slim Slow Slider:Free VST Side Chain Compression effect download here
Also available for free is the Side Chain Gate/Expander.
Slim Slow Slider: Free VST Sidechain gate and expander plugin download here