I found a really good free one called Mp3Gain at http://mp3gain.sourceforge.net/
I found it very easy to use, plus the way it levels the volume is much smarter than the other apps I waded through. Most of them just clip off the loudest sounds, but this one analyzes the entire song and adjusts everything accordingly. It tries to adjust the volume based on how loud it sounds to the human ear.
The changes can be easily backed out if you choose.
There's also a feature to adjust the tracks of an album so the same volume relative to the album
Just follow these steps:
1. Select the files you want using the "Add Files" button.
2. Run "Track Analysis" to check each files volume.
3. Run "Track Gain" to change them all to the level you choose as the "target normal volume".
Let me know if you would like more info.
Hope this helps !