TVTonic
I'm constantly on the lookout for new ways to extend my Media Center. jkOnTheRun tipped me off to TVTonic over the weekend. It's by far the best looking interface for adding subscription audio and video content from RSS feeds to Media Center. The interface supports any audio or video format you can play in either Windows Media Player or QuickTime, which covers almost everything. A nice list of pre-populated channel selections is included for browsing, although you need to subscribe to get any of the actual videos. Add your own favorites to the list of channels, or make your own video channel and add it to the collection.