MP3Gain

You know how our MP3s all play at different volumes? MP3Gain analyzes your MP3s and determines which ones are louder/quieter and sets a hidden bit in the MP3 that tells your player how much volume to add or lower. Once you run MP3Gain on your MP3s, each one will sound just as loud/soft as the previous one and you can get back to doing what you were doing instead of constantly adjusting the volume.
For you audio geeks out there, it simply adds an entry to the ID3v2 tag that tells it "play at -6.5 db" so it doesn't compress, clip, or otherwise alter the audio data in any way.
MP3Gain