OpenELEC

OpenELEC (short for "Open Embedded Linux Entertainment Center") is a free and open source, small and very fast booting, embedded operating system. OpenELEC is a complete media center software suite that comes with a pre-configured version of and third-party add-ons with retro video game console emulators and PVR plugins.

OpenELEC is also designed to be used with flash based drives, like compact flash, USB drives, or SSDs.

OpenELEC works on most computers you can install on, even the Raspberry Pi and the Apple TV 1 (silver only).

Main features
OpenELEC includes all features that has to offer.

Installing OpenELEC
A detailed guide about installing OpenELEC can be found at their wiki page: Installation

Updating OpenELEC
A detailed guide about updating OpenELEC can be found at their wiki page: Updating