Open Embedded Linux Entertainment Center (OpenELEC) is a small Linux based Just Enough Operating System (JeOS) built from scratch as a platform to turn your computer into a 
Kodi (previously XBMC) media center.
OpenELEC is designed to make your system boot as fast as possible and the install is so easy that anyone can turn a blank PC into a media machine in less than 15 minutes.
Movie Organizer and Player
Organize your movie collections. Play your videos and scrape relevant info, subtitles and fanart. All video info is shown via themes you like.
Picture Browser
Watch all your photos with OpenELEC manually or use a handy slide show. Zoom and/or rotate your pictures for the best presentation.
TV and Personal Video Recorder
Watch your favourite TV channels. Browse through the TV listings and record your favorite TV broadcasts. Watch recordings when you like.
TV Show Management
Manage your TV series and keep track of your favorite episodes. Descriptions, actors and other info available via online scrapers.
Music and Audiobook Player
Audio files in many different formats can be listened to via the Music section. Artists photos & album covers are available as Kodi fanart.
Easy expandable with Addons
OpenELEC can be enhanced via numerous addons, which cover a wide range of areas such as skins, tools and also even domotica.
Comments and Reviews
If what you want is a fast booting, easy to set up media centre then you really should look no further. With a 5400rpm drive I boot from cold in about 12 seconds. The support on the forum and IRC channel is amazing, too. My keyboard driver was disabled in the linux kernel so I went into the IRC channel, said which one it was and 24 hours later it was working! And at a size of just over 100Mb, I would never again use anything else on my media systems.