Extenders allow you to make take your HTPC to any room of the house. All of your content will automatically stream to an extender making media center the best HTPC around. The problem is that there aren't that many good extenders out there.
Xbox 360: This is the only extender that I can recommend. For $200 not only do you get an extender, but you also get a great gaming machine. The User Interface that is displayed on the Xbox 360 is identical to what it is on you Windows 7 machine. It connect fast, plays all of your media, streams live tv. It even has an IR port built in so it will automatically work with your Harmony remote. The newer models are much quieter and more energy efficient, it is by far the best extender out there.