Connect 360 is a Mac OS X preference pane that lets your play/view content from your Mac OS X computer on your Xbox 360. It uses iTunes to get the list of tracks and playlist, and iPhoto for photos. You specify a directory to indicate what videos should be shared.
The demo worked for me, though I only used it to view videos. The demo will let you view a good number of files.
I would love to use this to view all of my content on my TV. This would allow me to remove the Mac mini from the AV rack and keep it in the closet. Unfortunately, the Xbox doesn't support all of the video formats that the Mac mini can, so I would have to transcode many of my videos.