XBMC Media Center is a free cross-platform media player and entertainment hub software for Linux, Mac, Windows, as well as the Xbox game-console. XBMC is capable of playing back all popular video, audio, and picture formats out-of-the-box.
ISO image for creating a bootable CD to try XBMC. Also installable to a USB flash drive.
GNU General Public License sourceforge.net/projects/xbmc