There are so many Media Centers available for any kind of platforms. Most of them come with free GPL, GNU license.
Following are some famous Media Centers:
•    Windows Media Center(Not for free)
•    Boxee (fork of XBMC, coming with some customizations)
•    XBMC (Free /Open Source)
•    Moovida (Free /Open Source)
•    Myth TV (Free)
•    Plex (Free /Open Source)

Also there are Media Center Operating Systems such as Windows Media Center Edition, GeeXBOX for Linux...Etc.
 
Boxee is great and awesome media Center . I also used it as my default Media Center. Automatic detection of proper subtitles for movies is the feature I like very much.

o    Boxee

 
Boxee is a cross-platform freeware media center software with a 10-foot user interface and social networking features designed for the living-room TV.

Boxee is a fork of the free and open source XBMC media center software which Boxee uses as an application framework for its GUI and media player core platform, together with some custom and proprietary additions.
Boxee is still under development and is currently only available as alpha releases for Mac OS X, Windows, and Linux, as well as Apple TV, for computers with x86 architecture processors.

It will be the most popular Free and Open Source Media Center. Because it is fully armed with Social media Contents and Options like Twitter, Facebook etc.

Watch this Video : You can see all of the features that Boxee has.





o    Windows Media Center



Windows Media Center is an application with a 10 foot user interface designed to serve as a home entertainment hub for the living-room TV.
It is included in Win Xp Media Center Edition, which must be installed as an operating system. It is also included in premium editions of Windows Vista and Windows 7 .

It is designed to be controlled by special remote controls which prominently feature the Green Button, but can also be operated with a mouse and/or keyboard.
Playable Media: Media Center plays the computer user's pictures, videos, and music from local hard drives, optical drives, and network locations.

o     XBMC


XBMC (formerly named Xbox Media Center) is a cross-platform free and open source media player and home entertainment system application software with a 10-foot user interface design for the living-room TV.

It was originally created for the first-generation Xbox game console, but is now available for the Linux, Mac OS X, and Microsoft Windows operating systems.
Support Media : XBMC supports most common audio, video, and image formats, playlists, audio visualizations, slideshows, weather reporting, and third-party plug-ins.


o    Moovida






I have checked moovida on Ubuntu 9.10 and it is very well performed. But it is lack on keyboeadr and touchpad inputs.

Moovida (formerly Elisa) is a project to create an open source cross platform media center solution, with a 10-foot user interface design for the living-room TV, to be used with a remote control.
Primary development and deployment platform is Linux and Unix operating systems(Now also support Microsoft Windows and also hope to support Mac OS X in the future).


Moovida runs on top of the GStreamer multimedia framework and takes full advantage of hardware acceleration provided by modern graphic cards by using OpenGL APIs.


o   Plex



Plex, is an open source media center for Intel-based Apple Macintosh computers, it has a 10-foot user interface design for the living-room TV. It was forked from XBMC media center software on May 21, 2008.

The framework allows users to manage video, photos, podcasts, and music from a computer, optical disk, and the Internet using an Apple remote.


Most of the Media Centers Use 10 –Foot user interface for there develops. A 10-foot user interface is a software GUI (graphical user interface), designed for display on a large television (or similar sized screen), with interaction using a regular television-style remote control.
10-foot means menus, buttons, and fonts and so on.