MP3Gain analyzes and adjusts mp3 files so that they have the same volume. The program does not just do peak normalization, as many normalizers do, but instead attempts statistical analysis to determine how loud the file actually sounds to the human ear. The changes MP3Gain makes are completely lossless, there is no quality lost in the change because the program adjusts the mp3 file directly, without decoding and re-encoding.
Further explanation of Replay Gain (the statistical analysis done by MP3Gain)
http://replaygain.hydrogenaudio.org
Current discussions in the "General - (MP3/MP3Pro)" section of the forums
http://www.hydrogenaudio.org/forums/forumdisplay.php?s=&forumid=17
compiled Linux version
http://home.hccnet.nl/h.edskes/mirror.htm#mp3gain
WebAttack
http://www.webattack.com/Freeware/gmm/fwmp3.shtml
Freewarec -> Multimedia -> Misc. MP3 Tools