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.
It needs player support to work. Non-supporting players will play back the files without problems, but you'll miss out on the benefits. Nowadays most good players such as Winamp or foobar2000 or recent XMMS are already compatible. If your player isn't, consider upgrading to one that is.
Some music players such as foobar2000 have built in ReplayGain scanning functionality, which can be used instead of the commandline tool below. Linux users may want to check the packages in their distribution. A version of vorbisgain with additional patches may be available.