Many many plug ins in the sea, and many of them are useful. We have good plugin equalizers, limiters, dither engines.....but really good compressor plugin that will compete with hardware is an ongoing challenge. To date the Algorithmix Splitcom seems the best of the two channel linkable sort, and the Massenburg MDW is an excellent mono compressor.
The latest entry is a plugin compressor by Elysia (the Alpha)....can a $200 plugin hold it's own.? The hardware version is $10k! http://bit.ly/dpJ44Q
It certainly has all the flexibility you would ever desire including side chain filtering, linking/unlinking, wet/dry mix (read parallel compression) and more.
In my tests thus far I am fairly impressed. It seems to lack some of the edginess that comes with less than ideal detector design. I'll have a better idea after another week of use and abuse.