I think this shouldn't be a feature of a tagger, more a player based part. Some good player have this. I checked over years all players (Windows and Linux), take a look at foobar2000 (http://www.foobar2000.org), really the best player. Complete configurable, based on a small set of basic function, scripting language and a lot of plugins. Large community and it's free.
I strongly disagree! Since the Replay Gain software actually analyzes a songs apparent loudness adds tags telling the player how much to automatically adjust the volume up or down based on the other songs being played before or after it so everything has the same apparent volume.
So basically it is just a software algorithm that calculates a tag value based on a mathematical waveform and sets a new id3 tag in the file.
Now the software players watch for this tag and adjust the musics max volume up or down accordingly.
I would VERY MUCH like to see this in the next couple of versions of Jaikoz since there is no good replay gain software on the mac at this point and not everyone uses itunes (which has apples version of replay gain in it called sound check).
It is basically just adding a new mp3/mp4/aac/flac tag to files and I do believe that Jaikoz is an audio file tagger. Am I right?