I've just discovered two free compressor that are really good. I wasn't very much into compression, basically i thought that what a digital comp can do you can do with more precision using envelopes and volume automation, and maybe a transient designer. Main reason to use one imho is for the sound, and generally speaking you can't beat hardware here.
But try to put one of those little beast on your master or drum bus and fiddle a little with the settings, i'm sure you will be amazed.
Here they are:
http://vladgsound.wordpress.com/plugins/molot/ this is not very transparent but add amazing coloration
http://www.tokyodawn.net/tdr-feedback-compressor-2/ this one get raving comments on gearslutz, seems it's up to par or even better than some commercial ones
Also, i've heard good thing about those plugins http://varietyofsound.wordpress.com/downloads/ . Unfortunately i haven't tried them because i'm on a full 64 bit workflow and those are 32bit.
Another one worth mentioning is Klanghelm DC8C. It's commercial but it cost only 20 €. Basically it can sound similar to the tdr feedback 2, but you can also dial more extreme and coloring settings. It can be used in feedback or feedforward mode, and it has a preattack knob similar to a predelay. You can imagine what you can do to drums with this thing. If you have some spare cash, buy it, you will not regret it!
Hope you find those useful!
EDIT: it would be interesting for some of you that have an hardware one to see how these compare. Maybe you can create some waw samples and put them here for a blind test?