XBMC is the open source app that started off as "Xbox Media Center" (designed to be run on the original Xbox hardware), but has now blossomed into a full-featured media center that is usable on your Apple TV or Mac. Thanks to an app, you can use your iPhone as a remote as well.
The new version 9.11, a.k.a. "Camelot," has far too many new changes for us to list in their entirety here, but there's a revamped (and good-looking) user interface with increased skinning capability, updated support for different subtitles and video formats, new movie database scrapers for picking up information, and specifically in Mac OS X, support for the very popular Logitech Harmony Universal Remote. The devs say they're excited to get this one out the door, if only because it means they can move on to bigger and better very soon. Kudos on the release (during the holiday season!), and if you're an XBMC fan, have at it!