Well I'm more than a bit new to all of this, but I use the compressor which comes bundled with ntrack. You need to get your credit card out though as without registering it cuts all sound every ten seconds. 
I've used it to beef up the drums and bass, and as a new toy I obviously used the maximum settings! After I mixed down and played the track back it was only the back wall of my house which stopped me flying backwards through the air, so I recommend it ! 
I've heard there are other good free ones available though. 
Cheers,
Cazz.