OpenELEC for Raspberry Pi

Open Embedded Linux Entertainment Center (OpenELEC) is a small Linux distribution built from scratch as a platform to turn your computer into an XBMC media center. OpenELEC is designed to make your system boot fast, and the install is so easy that anyone can turn a blank PC into a media machine in less than 15 minutes.

You can get OpenELEC for Raspberry Pi from here and also the instructions how to install it.

More information about OpenELEC from the main site