gst123 is a GStreamer-based command line audio player in the style of mpg123 and ogg123. The advantage over these tools is that gst123 can play every file GStreamer can, so mixed music collections with FLAC, Ogg, and MP3 files are no problem. Newer versions of gst123 additionally provide support for playing video files, while the ability to use gst123 without X11 display has been preserved.
Beets is a media library management system designed for obsessive-compulsive music geeks. It catalogs your collection, automatically improving its metadata as it goes using the MusicBrainz database. It also downloads cover art for albums it imports. Then it provides a bouquet of tools for manipulating and accessing your music. Beets also includes a music player that speaks the MPD protocol, so you can play music in your beets library using any MPD client.
Pimp is a highly interactive and programmable music player. It can be controlled with an mpd client or via a Python interpreter. Files can be played directly, and file paths are recorded in a database. Files are identified by a "fingerprint" which allows the user to identify an audio file despite file moving and tag modification. Pimp aims to be a player framework with a really small core. All features are implemented in extensions. For example, a lot of player events such as play, seek, and pause are logged in the database.