I've been using this free open source app MP3Gain recently. This app normalizes the volume of an mp3. For me, this is great, since I had a yearly playlist for the year 2006 on my mp3 player. A few songs were real loud or soft, so I had to frequently change the volume on my speakers.
I tried a test of using MP3 Gain on that set of mp3s. The test worked in that I don't have to adjust the volume anymore.
If you use this app let me know of your experiences with it.