Download Side Chain Compressor:

Simple side chain compressor

The free Side Chain Compressor plugin by Slim Slow Slider offers simple side chain compression (if your host allows it) both in mono and stereo. It features soft and hard knee, and the usual compressor control buttons like threshold, ratio, attack, and release. Of course the Side Chain Compressor VST plugin can also function like any other "normal" compressor without side chaining the audio signal.