GeeXboX - Standalone Linux Media Center on a LiveCD
The folks at Slashgear recently posted an interview with Benjami Zores, one of the core developers of the embedded Linux distribution knows as GeeXboX.
In summary, GeeXboX, which incidentally has no affiliation or tie in with the Microsoft console, is a fully operational Media Center solution available on what's known as a LiveCD. It works on both x86 and PowerPC based computers and is as easy to use as dropping a CD in a PC since no installation is required.
GeeXboX plays DVDs, video files, audio files, live and recorded TV, and allows users to schedule television recordings - all characteristics of a respectable Media Center platform.