bluefuze - it claims not to rewrite the files. Judging from the speed, I'd say it was honest. If you feed it just one file as a test, you'll see it takes about 20 seconds to analyze it. During analysis, no changes are made. Then you set the db level (usually 89) and click the "gain" button. It adjusts the volume on the file in less than a second.
Here's the faq link:
I was nervous too - I backed up all my files before I turned it loose on them. But I was very pleased with the results, and it really did the job. Listeners no longer have to dash for the volume control on a song change
(Added - There are DSP plugins that do the job, but you have to use a stacker so you can still use the SHOUTcast DSP too. I opted for fixing the problem at the source... the files)