Raspbmc is a distribution for the Raspberry Pi that turns it into a media center you can use to play movies and audio stored on USB media attached to the Pi, or you can stream audio and video from other devices such as iPads that are connected to your home network. Raspbmc is based on the successful XBMC project, which started life as a project to use Microsoft Xboxes as media centers. However, it’s available on a wide range of platforms.
With the low price of the Raspberry Pi, it seems likely that a lot of them will find their way into little boxes next to the TV—especially now that many TVs have a USB port that can supply the Pi with power.