I recently came across Boxee, a media center application that runs on a number of platforms, including the Apple TV. The Apple TV is a pretty capable box on its own, but only within Apple’s garden of iTunes. There were only few options for viewing content outside of what iTunes supports.
Boxee changes all that and brings content such as Hulu and the Comedy Central video library to your TV. It is pretty easy to install on your Apple TV, and even though in only in alpha release, works pretty well. There are some bugs here and there, but being able to view nearly any video from the Internet on my TV is a great feature. Video quality is certainly not HD, but is watchable.
Chris Breen at Macworld has a good set of instructions for how to install it on your Apple TV, and more information about Boxee is here.