Clementine is a music player written using the Qt4 libraries. It begun as a fork of the long-dead Amarok player. It was last updated in 2016. It's over, Clementine is dead.
The music player Strawberry is a fork of Clementine which is based on the more modern Qt5 libraries. It actively developed and maintained.
Stand-alone music players:
Music Player Daemon clients:
mpd is a database-oriented music player daemon which can be controlled by numerous front-end programs.