Heel Audio have released Duck EQ, a free equalizer plug-in for OSX that operates dynamically via the sidechain input.
Here's what they have to say about it: "What is it good for?
Ducking has many applications in mixing and broadcast, especially for managing tracks with overlapping frequency content, like Bass and Kick Drum, Vocals and Electric Guitars etc. DuckEQ’s integration of EQ means it can duck only specific frequency bands, as well as normal ‘broadband’ ducking. Apart from ducking, there are many other applications that DuckEQ can be used, for example instead of ducking a signal, one could apply gain to the EQ bands and “EQ-Gate” .
What’s the difference to using a compressor? In many cases a Compressor with a SC input can do the job, however specialised duckers have more control over the gain reduction. For example, when ducking a guitar from vocal, a compressor will reduce the guitars more on loud words and less on quiet ones, which is the opposite of what is desired. A ducker reduces the gain the same amount every time."