x10mp3 is a plugin for XMMS to control it using X10's Mp3Remote, which comes with Mp3Anywhere, Mp3Anywhere 2000, or DVDAnywhere. It has all the functionality of X10's BOOM2000 software for Windows, plus other handy features like muting, skipping around inside tracks, and balance control.
xmms-alarm is a plugin which allows XMMS to be used as an alarm clock. It features alarm start time, sleep time (so the alarm will stop after a while), volume fading time, loading a seperate playlist for the alarm, executing a shell command when the alarm goes off, and the option to choose a starting volume when fading.
xmms-crossfade is an output plugin for XMMS. Its main feature is automatic crossfading. It also removes the gaps between tracks, to play whole MP3 albums without any audible interruption. xmms-crossfade supports OSS via a builtin driver, and any other platform by using existing output plugins. As of version 0.3.9, the Audacious media player is also supported.
mpg321 is a Free clone of mpg123, a command-line MP3 player. It is designed to be a drop-in replacement for mpg123, and therefore its interface has been designed around that of mpg123, without using any of its code. mpg321 has been designed for use with frontends such as gqmpeg, although it is just as useful on the command-line. Unlike mpg123, it supports ESD and ALSA output without recompiling, and it does all MP3 decoding with only fixed-point math by using the mad MPEG audio decoder library.