Let me recommend Winamp,
the best and most known free mp3 player available. Winamp
is currently in version 5. However the best sounding player was Winamp
2.91... Of course it is very hard to find now. The current version will
do fine though.
As you can see from the snapshot, the interface is very intuitive and
the main components of the program (player, EQ, ...) can be easily arranged
as separate windows or linked together making customization a simple
Additionally, the interface is skinable enabling full customization
of the look and layout of the player.