GeeXboX is a full operating system, running under Linux and based on the excellent MPlayer. No need of hard drive, you just have to put the GeeXboX bootable CD into the CD-Drive of any pentium-class computer to boot it. Moreover, GeeXboX is a free software, created thanks to many open-source software. This means that everyone can modify it and build his own release of the GeeXboX. You may wonder why you could have to boot on another operating system to play your media files, but just think about the Mini-ITX plateforms like VIA Epia/Eden or Shuttle barebones.