Beatbox is a new music player currently under development by the elementary team. As of now the main features include Last.fm integration, smart playlists, simple UI, album view browser, music search, etc. More features will be added in future.
The official Beatbox PPA has not been updated for quite some time. However, there is now a new PPA created by Gusions which will be updated more frequently. You can add this PPA to your sources by running the commands below but note that these packages are development builds and are far from being stable (for Ubuntu 11.04 only)
sudo add-apt-repository ppa:gusions/bbox-pr
sudo apt-get update
sudo apt-get install beatbox