Mac OS X only: The latest version of the free Plex Media Centre for Mac now includes iTunes and iPhoto support, iTunes visualizations, TV theme music, and the ability to play songs you've purchased from the iTunes Store. This tight iTunes/iPhoto integration comes in part from the Plex Media Server, which makes your songs and photos show up inside Plex while running in parallel. The Plex developer explains:
The Plex Media Server is a standalone program that runs alongside Plex (or alone on any machine, it's a Universal Binary). It serves up media from your iLife applications (iTunes and iPhoto today, Aperture and Lightroom shortly). Plex communicates with the Plex Media Server on the local machine, on your local network, or even across the world over the Internet. This means that you can play your friends' iTunes playlists or browse their podcasts or photo albums.
The Plex Media Centre is a fork of the XBMC project, which also offers a Mac version. In fact, XBMC Atlantis' Mac version also includes iTunes and iPhoto support; compare our Plex screenshot tour to the Atlantis tour to see the differences between the two projects, which share the same code base. Plex is a free download for Intel Macs running Leopard only.