My second Raspberry Pi is connected to my TV running Raspbmc to play video files. I liked the idea of a custom ambilight installation described by _nadnerb so I bought some WS2801 LEDs and installed Boblight. The LEDs are connected by SPI to the raspberry. Boblight is then able to control the LEDs based on the output.
You can see the mounted LEDs and the Boblight config in the following images: