One that a lot of people like is Spitfish. Its from digitalfishphones and comes in a free bundle.
The compressor in that bundle is nice too (for vocals even).

http://www.digitalfishphones.com/main.php?item=2&subItem=5

On Spitfish
Tune lets you narrow in on the freq
Sensitivity lets you adjust how much it grabs (kinda like threshold)
Depth lets you adjust how much it cuts what it does grab

It may be worth checking these plugins out. Blockfish and Spitfish are in the plugin list here.

I think in PG version it is
frequency adjustment,
the 'threshold' is how loud it has to get before it kicks in
the release is how soft it dies down to before the de-esser relaxes and stops clamping it down

That 'ratio' (or depth) adjustment is the difference

I *think* there is a section in the manual that explains the DX plugins.

Ozone4 does not work well in the track FX slots, it is known to have issues on various softwares as it is not designed to run there. Same with some other mastering software I have. There is no de-esser in Ozone4 anyway.

I do not work here, but the benefits are still awesome
Make your sound your own!