Hulu is TV show and movie streaming service. It is a joint venture of NBC and Fox so most of the content on Hulu is from these two. Definitely a must on any HTPC. If you would like to watch shows that were aired a long time ago you will have to subscribe to Hulu Plus, their premium service.
Hulu Plus Windows 8 app: http://apps.microsoft.com/windows/en-us/app/hulu-plus/ad77e4ed-0398-4f5b-a41e-88973007dca1
Hulu Plus iPhone app: https://itunes.apple.com/us/app/hulu-plus/id376510438?mt=8
Download Hulu desktop: http://www.hulu.com/labs/hulu-desktop