The funnily named DeaDBeeF is a simple music player in the vein of Foobar2000. It can play MP3 and other audio formats and comes with tons of plugins to customize its looks and operation. It has been ported to work on all the 3 major operating systems.
Install: For Linux, I downloaded the .deb file from here and installed it.
Customizing UI: By default, the player displays just a seeker, controls and playlist. To customize the UI, choose View -> Design mode. Then right-click anywhere to add UI boxes and then replace them with the UI elements you want. For example to display album art under the playlist: right-click and choose to add an extra item in the parent UI element. Then right-click in that new box and replace it with Album Art component.