The Apple TV wasn't released long ago, but the response after the release came from Google. Google is planning to launch TV services, later this year in the U.S. and around early 2011 to many other countries. The Google TV platform is set to be built in to Sony TV's coming later this year, as many TV companies, have teamed up with Google to provide this service. The Google TV does not come in it's own design form factor, instead you will need to purchase a set top box. Such as one which was by Logitech. The Google TV platform is completely software based and you would require your own piece of hardware to run it. 

The Apple TV on the other hand, features a form factor, and a pretty decent user interface. With it being cloud based, it is much cheaper and is less prone to having content pirated. Though the idea of having to only rent movies is not so great, and it isn't possible to hook up an external drive. 

The Google TV will probably aim to be cheaper than the Apple TV while running on a set top box, it might also have the capability to hook up an external drive, to download content. I am not sure as to how the user interface will be on this, but from the looks of videos, it seems quite nice and easy to use. 

The Google TV will most likely come out later this year, we will have to see what the rivalry will be like between the Apple TV and Google TV.