(lookout, long complicated answer below)
You do not want to normalize your files using any sort of program such as this. It will adversly affect quality.
If you do need to normalize any audio, you should do so with 24bit software that is designed to transparently perform that function.
(such as soundforge, wavlab, nuendo, protools, etc....)
When in 16 bit, any sort of volume/gain editing requires the program to perform a complicated set of algorithms and the result doesn't add up evenly, leaving artifacts.
This can be percieved as dullness or lifelessness with the recording. The more edits or actions you perform, the more this is compounded.
It's akin to a large number that is divided by a smaller number and leaves an uneven remainder.
When doing this type of editing, you need to import the 16bit into 24bit program and do the edits in that realm. The errors are dramaticlly less and the chance of these nasty artifacts are greatly diminished.
As a general rule, you should be very sparing with volume adjustments. If needed, use trial and error, and the undo function to find the right level/setting, then do it once and leave it alone.
You will need to dither the 24 bit file back down to 16bit afterwards. After the dither, there should be NO MORE "destructive" edits performed on the audio. If you edit after a file has been dithered, it will scramble the bit quantatization, or in other words, it will make it sound worse.
Most of those above things can and will happen if you use a program like itunes, nero, winamp, foobar, flac, or whatever to normalize your files.
Cheers, Phil