Boxee is an interesting new media center application for the Mac based on the XBMC. Separate from the OS X branch of XBMC (now called Plex), Boxee focuses on a social networking experience by allowing you to get (and give) recommendations to friends who are also on the service. You can also share your media activity with friends and vice versa.
Once installed, Boxee scans your hard drive for videos, music, and pictures. It also interfaces with various streaming sources, both for audio and video, including last.fm, revision 3, YouTube, etc. Even when playing local content it will download things like album covers and descriptions from the internet. All in all, Boxee looks like an interesting entry into the media center competition and its social networking features set it apart from more straightforward implementations like Plex.
Boxee is presently in a closed alpha test, but they're expecting to open a public beta by the early autumn. You can sign up to be notified when the open beta is available. Since it's based on XBMC it's Intel Mac and Leopard only.