www.replaygain.org
Basically, it adds a couple tags in the comment of the Vorbis file. They are used by the player to normalize (equal the volumes) of tracks. Single/Radio analyses a single track and generates it's gain (amp adjustment). Album takes the group of files and analyses them as a whole to maintain the volume difference in the entire album as a whole. Not too many player support it currently though. Winamp and some other do.
Or something like that...