VorbisGain is a utility that uses a psychoacoustic method to correct the volume of an Ogg Vorbis file to a predefined standardized loudness.
It is meant as a replacement for the normalization that is commonly used before encoding. Although normalization will ensure that each song has the same peak volume, this unfortunately does not say anything about the apparent loudness of the music, with the end result being that many normalized files still don't sound equally loud. VorbisGain uses psychoacoustics to address this deficiency. Moreover, unlike normalization, it's a lossless procedure which works by adding tags to the file. Additionally, it will add hints that can be used to prevent clipping on playback. It is based upon the ReplayGain technology.
The end result is that playback is both more convenient and of higher quality compared to a non-VorbisGain'ed file.