Oldie, but goodie- XboxMediaCenter is a free open source (GPL) multimedia player for the Xbox™ from Microsoft. Currently XboxMediaCenter can be used to play/view most popular video/audio/picture formats such as MPEG-1/2/4, DivX, XviD, MP3, AAC, JPG, GIF plus many more less known formats directly from a CD/DVD in Xbox DVD-ROM drive or of the Xbox harddrive, XBMC can also play files from a PC over a local network and even stream media streams directly from the internet. Link.
