XBMC Media Center (XBOX Media Center) is an award winning free and open source media center which is also multiplatform. It looks stunning and is also feature rich. Apart from being an excellent multimedia player, it has of features like visually appealing user interface, playlist playback, picture viewing, some really good skins, and even third party softwares and addons. XBMC media center is released under GNU General Public License.
Eventhough its still in the devolopment phases, it is worth a try. We have detailed the different steps in Installing XBMC Media Center in Ubuntu 8.10 and also the list of features included in XBMC with screenshots. XBMC Media Center, with its sleek and easy to use interface, is going to give its proprietory counterparts a run for its money. There is even a Live CD version of XBMC, called XBMC Live.