MP3Gain is a program that equalizes all the songs in your Library so they're the same volume, since I get my music from...various sources they're usually different volumes by default and since going through iTunes and doing it manually would take hours. This program just makes them the same volume automatically that way if I have it on Shuffle I don't have to turn it down or up.
I tried Audacity, but I didn't like the look of it, I also tried Nero WaveEditor, but I usually use that for something else. I also tried GoldWave
MP3Gain is amazing, but it hasn't been updated in almost 2 years.