Well there is lots.
Mplayer --- movie player
Xine --- movie player
Totem -- movie player
Xmms -- audio player
Zinf -- audio player + advanced playlist manager.
Whole bunches of stuff. Open source means that anybody can take any program, change it and release it as a different program. Many variations.
I am using a thing called mpd, music player daemon. It acts like a service, running in a background. It's only job is to play music. So when you log in and out, open and close programs all it does is just keep on playing music.
In order to control it you use one of many clients. They have clients for Gnome/KDE and even web stuff and command line controllers. Basicly it just plays from playlits. Very basic, but it's great for when "You Just Want To Listen To Music" (tm).
To see what stuff has to offer, check out
sourceforge and
freshmeat tp see various offerings.
music player daemon