XMBC is a free and open source (GPL) software media player and entertainment hub for digital media. Some people are currently working on porting it to the Raspberry Pi board.
They posted a video demo on their forum and I uploaded the video to YouTube for those interested in seeing the progress. The video just shows XMBC booting, then accessing the menu and showing the Videocore GPU is detected. This is still work in progress, but it looks promising.