Parent: Software
Exaile is a music player for Ubuntu. It aims to have the same functionality as KDE's Amarok, but based in GNOME instead.
Key Features
Screenshot
Installation
Hints and tips
Other music players
Amarok - KDE's default music player
Banshee - Music playing and managing for GNOME
Rhythmbox - Ubuntu's default music player
Further reading
Restricted Formats on Ubuntu - Information on configuring support for non-free formats, like mp3.