Just opened up Windows 7 Media Center today and spotted a new icon in the TV section - MSN Video Player. Clicking this installs the Silverlight-based player (after a Silverlight update), giving you free access to hundreds of free streaming TV shows (haven't counted them all yet, but there are 112 shows in the Sci-Fi section, including 55 Doctor Who episodes). There are five categories: Comedy, Entertainment, Documentary, Dram and Sci-Fi, with predominantly UK content. Each show starts with a 15-second pre-roll advert.
According to this Guardian article from March, MSN has bought around 1,000 hours of programming, but unlike the BBC Iplayer, it doesn't have a catch-up service.