TVTonic plugin for Windows Media Center allows you to download online video content in WMC through RSS. There’s also a video directory or guide, which categorizes popular video or vodcast feeds and allows for direct integration in your TVTonic. If you want to add your own feeds you’ll need to add the RSS link manually in Internet Explorer (no Firefox) on the TVTonic website.
The GUI is nice and the plug-in integrates very well with the standard WMC interface even though it can be extremly slow sometimes. This probably depends on your connection speed since everything is downloaded live. The plug-in automatically downloads every video in the available RSS feeds so people with a bandwith limit, beware!
More info: TVTonic