I've just installed a great set of audio plug-ins on my system and must say they work terrifically. Called blockfish, spitfish and floorfish from digitaldishphones.com, they will compress, de-ess and expand/gate you audio. And to sweeten the deal, they're extremely easy to use with easy minimal interfaces that make sense. I used it successfully to sweeten a voice recording I did this evening. Give it a try, the price is right.
Comes in Apple AU Units or VST flavors.
UDPATE 12/9/07: Doesn't seem to work on Intel Macs, maybe a universal version is on the way?