The Extended Module Player is a command-line mod player for Unix-like systems that plays over 90 mainstream and obscure module formats from Amiga, Atari, Acorn, Apple IIgs, C64, and PC, including Protracker (MOD), Scream Tracker 3 (S3M), Fast Tracker II (XM), and Impulse Tracker (IT) files.
AmpJuke makes it possible to manage and stream your music online to several clients connected through a LAN or the Internet. In addition to streaming music, it can fetch "metadata" (album covers, images, lyrics, and track "fingerprints") automatically using various Web services such as last.fm and the.echonest. You can scan and import tags from as many tracks as you like, use favorites, and customize personal settings. Its focus is on easy navigation as well as speed.
m9u is a music server, similar to mpd or xmms2. It sits in the background, waiting for commands to be delivered via a 9p file system. m9u embraces the Unix philosophy of doing one thing well. As such, it does not try to catalog or otherwise organize music, but aims for the more humble goal of simply playing it. A side effect of this approach is that m9u is trivially extensible to domains aside from music (for example, an image slideshow).
Decibel is a GTK+ audio player designed for GNU/Linux. It aims at being very straightforward to use by means of a very clean and user-friendly interface. It aims also at being a real audio player and, as such, it does not include features that are not meant to be part of an audio player.
SvOlli's Little Audio Related Thingies, or SLART for short, is a compilation of rather simple programs that are related to audio files: it consists of a player (Partyman), a CD ripper, a tag editor, and some other tools. These programs can communicate with each other via a custom IPC bus, so a newly ripped track can be automatically queued in the player, for example.