Libraries:

GTK 1.2.0 (The Gimp Tool kit) GTK's homepage (FTP site)
gdk-pixbuf 0.7.0 (Image library) ftp.gnome.org

Programs:

mpg123 0.59o www.mpg123.de (mp3)
Note: mpg123 0.59p or later required for streaming support.
ogg123 www.xiph.org (ogg)
xmp 2 xmp.helllabs.org (mod, etc.)

Playlists, with playlist editor.
Extensive support for options of mpg123.
ogg support (through ogg123).
mod support (through xmp).
radio tuner support through Video4Linux.
Shuffle, Repeat, and Repeat 1.
Shuffle and Repeat modes can be saved in a playlist.
Custom skins, including winamp skin support.
Built-in skin editor.
ID3 (v1) tag reading/writing support.
Ogg comment editor.
Volume Controls.
Basic file management (move/copy/rename/delete).
Drag and drop.
Command line control of playback (see GQmpeg --help).