Acustica TAN Compressor
 
Here's a freebie that's certainly worth a download -- once you have an Acustica account set up, you can give it a spin. 
 
Until very recently, Nebula-based compressors weren't very impressive. EQs, preamps, tape machines -- these were capable of fantastic sound, but the quick dynamic action of compressor hardware seemed to elude what Nebula was capable of. This TAN compressor, however, provides a taste of what's possible now.
 
I'm getting very good tight and punchy results on drums with this. You do have to push it pretty hard, and the GR meter is best ignored, but it does a great job. I can't think of another free compressor that I'd rather use when it comes to sound quality.
 
As with many Acustica plugins, the CPU hit is a little high. But stability seems better than it used to be with these things. It looks like Acustica is getting close to the point where other plugin vendors will have to pay attention and step up their game.
 
Cheers,
Eddie
 

I have downloaded and installed this too and I also agree it is excellent.  I am also finding it also does a very fine job doing light conditioning duties as well.  It also can sound great in this mode too.  Just adding glue.
 
Other very good free compressors and things are the Tokyo Dawn Labs plug-ins as well. They are so transparent I have used them in important mastering situations many times and got excellent results too.  It is TDR Kotelnikov. TDR feedback Compressor II is superseded by this but it still available.
 
Some of the paid Acustica stuff looks pretty impressive as well.