Selig's Leveler (formerly known as the Curve) is an intelligent processing tool capable of evening out tracks with incredibly disparate dynamic levels without raising any background noise present in the recording. I'm not sure whether it's available for any other DAWs, but it is touted primarily for use with Propellerhead's Reason.
It's quite easy to use, with a deceptively simple user interface that provides a lot of processing options. The basic idea is that you apply it to a track, and then watch as the meter tells you the lowest volume (in dBs) present, as well as the highest. Then, you set the target level, and adjust the Curve control to balance out the two extremes. The wet/dry knob allows you to blend the signal to taste, so you can squash the signal to make the quiet parts as loud as the loudest parts, or lay back a little to preserve some of the original performance's dynamic integrity.
What's more, the Selig Leveler somehow manages to accomplish this without coloring the tone of the track, and doesn't raise the background noise level. I have used this rack extension on vocals that were recorded in less-than-ideal circumstances (mainly bright rooms with more reflection than I'd have liked), and although I ran a noise gate to kill the signal between the singer's lines, there was still a little bit of extraneous air audible between certain syllables. The Leveler managed to raise the quiet verse parts substantially without these elements becoming more pronounced. I don't understand how it manages to do this, especially without a sensitivity or threshold control, but it does. I've subsequently had a lot of luck performing similar cleanup on live drum tracks, and on certain bass tracks, I use the Leveler instead of a compressor to smooth things out.
In short, I would say that the Selig Leveler is an absolute must-own rack extension for any Reason user who tracks live vocals or instrumentation. With a little experimentation, it can absolutely help achieve a more professional-sounding mix.