These patches add replaygain support to the xmms-mad plugin, as well as dithering and a few minor modifs.
It reads replaygain info in APEv2 tags, as written by the mp3gain program. (use e.g. mp3gain *.mp3 to add track and album RG info to all files in a directory. Note some options make mp3gain also modify the mp3 data directly instead of just adding tags, see mp3gain's man page).

After installing the plugin files, make sure MAD is the active input plugin for MP3 (disable the other mp3 input plugin(s))


Full modified source

Compiled plugin files (should work on PCs for most linux distribs)

Debian package

Recent changes :

For more details about ReplayGain and mp3 stuff, check HydrogenAudio's Wiki page

Also consider using the xmms-crossfade plugin to get gapless playback
The latest version (3.6) features high-quality resampling  (resampling to 48kHz before outputting to soundcard improves sound quality noticeably for the many soundcards that only handle 48kHz internally and do internal (poor) resampling for signals with other sample-rates.
Just try playing high frequency sine waves (17kHz and up) on a SbLive!, and you'll hear obvious artefacts from the poor hardware resampling instead of a high-frequency tone.