Although in the recent past Banshee and Canonical had a little, something going on (in a bad way :) ), I even thought that Ubuntu will completely remove Banshee from their official support... but they've decided to stick to it and as a result Banshee is the default music player in Ubuntu 11.04 Natty Narwhal.
Rhythmbox is a GTK written GUI which uses the Gstreamer front-end and according to its developers themselves, it tries to "follow" the work of iTunes. Although almost all knows about it and the features... yet let me give you a simple list of its features nonetheless.
*. Music browser which is pretty easy to use and lets you manage a large collection of music libraries with ease.
*. Playlist support (embedded).
*. Plays MP3, AAC, Vorbis, WMA, Flac... basically, anything that Gstreamer multimedia framework supports, Rhythmbox can play.
*. Internet radio support (Last.fm, etc) + live streaming.
*. Few built in audio visualizations.
*. Transfer music to and from iPod, MTP, and USB storage media players.
*. Rip/Play and burn Audio CDs.
*. Podcast downloading.
*. Preview and free download (legal) music files from sources like Jamendo and Magnatune.
These just a few to mention. Since it support plugins, you can expand its features by simply adding plugins as well. If you use Ubuntu 11.04 Natty Narwhal and want to know how to install Rhythmbox, then open your Terminal and enter the below command. Enjoy!.
sudo apt-get install rhythmbox