Windows only - Our friends at Gizmodo AU have written up the release of Webguide - a free upgrade which lets Media Center users stream video and other content across the net.
"Webguide was bought by Microsoft and made a free upgrade to all Media Center users. It basically streams all video, music, photos and TV across the net. It even re-encodes HD streams to lesser pixel counts."
As Shay says, free is free! I have to confess that I haven't played around much with the Media Center in Vista. If you use it, let us know what it's like in comments.