I was ripping my whole music collection into wavpack the other day, and just read up on replaygain. I was wondering if i could implement replaygain on my already ripped files, without having to rip them again. Is there a utility that can just add that kind of metadata on without altering the other tags?
Thanks for your help!

Foobar2000 will do this for you.

How does this work? I am not familiar with the interface of Foobar2000. Is there a way to do this in winamp?

right click the files in foobar-->replaygain-->scan files as album (or per track)
But IMO theres no advantage of using winamp, i switched to foobar a long time ago and i don't want to miss its features anymore.

edit: grammar

1. Right click : Replaygain > scan selection as albums
2. no.

thanks very much for your help. I also have another question about replaygain: Does it alter the audio data at all, or is it just a tag telling a replaygain-compliant player what to do?

Just a tag, that can be removed, or re-scanned. It does not alter the audio data.

tag. its completely lossless.

Also, when you use ReplayGain, does it normalize to a uniform volume every time? i ask this because i have different albums at different volumes, and was wondering if separate uses of replaygain on these albums would make the albums the same volume relative to each other, or just normalize the tracks relative to each other.

Nevermind... i found the answer at replaygain.org. it does what i want, which is very nice!

I tried using your advice, and in foobar2000, i loaded all my files into the playlist, and did the following: right-click > ReplayGain > scan files as multiple albums. It worked very well, and now my music is normalized! the best thing is that i can just turn ReplayGain on/off in winamp if i need to. Thanks for all your help!

Glad we could help