XboxSpace has an interesting, if cursory, comparison of the open-source media player for the original Xbox, the Xbox Media Center (XBMC), and the Xbox 360's Media Center extender functionality. The feature list on XBMC is monstrous, supporting many of the most popular codecs, streaming from SMB volumes, iTunes streaming, and more. The major shortcomings are that it requires a modded Xbox to run and additional storage capacities are strongly recommended.
The Xbox 360's functionality on the other hand is notably sparse in comparison, featuring limited codec support, video streaming only from MCE, and a similarly small 13GB hard drive. They conclude, "In sheer terms of Media Center functionality, XMBC is superior to the 360 in every respect. It is an application that you have to use to understand just how good it is."
What is upsetting is that this functionality far surpasses Microsoft's own, but they refuse to allow even trivial concessions like DivX support. Let's hope future Dashboard updates, and maybe even Vista, expand the feature set.