There are a lot of multimedia system out there, but I l believe the following 3 to be the best!
We like to think of MythTV as the ultimate Digital Video Recorder and home media center hub. Think of it as a Free and Open Source alternative to Windows Media Center or Tivo. It started out in 2002, the open source community quickly made it grow into a powerful tool for watching and recording television. With its plugin architecture, it has been transformed from a mere TV viewing application into a full home media center suite, capable of managing your personal photo, video, and music collections, as well as keep you up to date with the weather, a ZoneMinder home security system, and much, much more.
XBMC is an award-winning free and open source (GPL) software media player and entertainment hub for digital media. XBMC is available for Linux, OSX, Windows, and the original Xbox. Created in 2003 by a group of like minded programmers, XBMC is a non-profit project run and developed by volunteers located around the world. More than 50 software developers have contributed to XBMC, and 100-plus translators have worked to expand its reach, making it available in more than 30 languages.
Boxee is the best way to watch movies, TV shows and clips from the Internet on your TV. You can buy a device powered by Boxee or build your own Boxee using a new or used computer. If it’s available on the Internet, then you can probably watch it on your Boxee – there are more than 40,000 TV show episodes and movies currently available through Boxee. Plus, you can play personal stuff from your network or hard-drive.