There are very few free compressors that support sidechaining. The SSS Side Chain Compressor makes it extremely easy to get the ducking sound made famous by french house producers like Daft Punk. Click the link below to download the SSS Side Chain Compressor.
SSS Side Chain Compressor – Free Windows VST
NEW OS X Version! SSS Side Chain Compressor – Free Mac OS X VST & AU (The download link is in middle of the page).