XMMS2 is a redesign of the XMMS music player. It features a client-server model, allowing multiple (even simultaneous!) user interfaces, both textual and graphical. All common audio formats are supported using plugins. On top of this, there is a flexible media library to organize your music.
MaGnuX MP3 Control Server is a server that allows you to control the playing of MP3 songs. It uses mpg123 to play the songs, and features playlist management, a builtin Web server, IP-based access control, and much more. It's written entirely in Ruby, which makes it extremely flexible and very easy to extended.
Webplayer is a Borges-based, Web browser-controlled MP3 player. Webplayer features a file browser that can load or recursively load from a directory of MP3 files. It also provides simple playlist control and volume control (tested on FreeBSD). Components of Webplayer can be launched into separate windows or used in one big main window. Webplayer incorporates portions of Yoshinori Toki's rmp3.