Xfer Records OTT is a great sounding 3 band compressor expander. The Depth controls applies the amount of comression while the Time control affects the attack and release both.
It's often use on the master channel or busses. I personally like it on bass sounds. I believe it can help combat "frequency masking," in lower frequencies which is import in getting a cleaner louder sounding mix.
I talk a litter more about frequency masking here.