Yougle is an addin which extends your Media Center so it can play online video/audio from sources such as Youtube, live365.com and some others. Their source list isn’t very extensive but they’ve got Shoutcast and Youtube which makes up for a lot.
Unfortunately for us x64 users there are still some problems with playing back flash content. According to the Yougle Website this has to do with the FLVSplitter not being 64-bit.
More info: Yougle