Source: SourceForge.net
MP3Gain Screenshot
This software's UI is simple but very functional. You can normalize MP3 files quickly and accurately because it uses a statistical analysis algorithm to adjust the volume of your MP3 files.
This Freeware is highly recommended for MP3 volume normalization.
There is also a version for MAC computers and an AAC volume normalizer.
Download Site: http://mp3gain.sourceforge.net/download.php