Effect: STSoft - STSideComp / Mac / Universal / Free

STSideComp from STSoft is a FREE fully parameterized compressor with a sidechain input stereo channel. It is designed as a VST effect for MacOSX and is specially created for Ableton Live.

Find other compressors here.

"You can use the plugin especially in modern music styles like House or Techno in order to gain the typical ducking effect, best known from French House." Via