"Open Embedded Linux Entertainment Center, or OpenELEC for short, is a small Linux distribution built from scratch as a platform to turn your computer into a complete 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."
http://openelec.tv/
Available for a number of platforms, including RPi.