mp3gain
mp3gain is a Replay Gain back-end for mp3 files.
aacgain is based on mp3gain and supports the same functionality. In addition to that it also supports adding Replay Gain tags to aac files.
mp3gain has a unique feature to modify the audio stream of an audio file. This way the mp3 player doesn't need to support Replay Gain and the volume adjustment still works. mp3gain does that without re-encoding the audio file and it adds correction data to the mp3 file so all changes can be undone.
You can enable the modification of the audio stream in the mp3gain configuration dialog by checking the "Modify audio stream" check box.
In order to revert all changes, use the soundKonverter Replay Gain tool. Add your files to the file list and click the "Untag tagged" button.
This will first undo all changes made to the audio stream and then remove all remaining tags.
Note: Using Replay Gain tags and audio stream modification in parallel might result in some confusion. At any point it should be possible to revert the modifications and calculate the volume adjustment again.