Be aware this is not a completely featured Media Center like some other sites reported before, the X-Box Media Player is an open source media player for the Xbox. Currently the XMP can be used to play supported direct file formats (AVI, MPG, MP3, ...ect.) from a CD-RW/DVD inside the Xbox DVD-ROM or of the Xbox hard disk drive, it can also play files from a PC/server over the network via the XStream Streamer/Server software.
The XMP does require a modchip installed in the console to work properly though; a new version is also in the works with a front-end that resembles that of Microsoft's own Windows Media Centre.