I previously had a Xbox1, that had XBMC on it, which allowed playing of divx media via network shares(no HD content though). Since then I upgraded to a Boxee Box which plays the HD content, is small, has a TON of features, Allows watching flash content from websites(comedy central, youtube, and a ton more...). it can play Netflix, and Hulu+. It used to be able to play Hulu for free, not sure about the current status of that. It has a list of TV shows and multiple sites that host the content(ie http://www.nbc.com/30-rock/video/
). It is a little more expensive than the other hardware boxes, but definitely has more features. I've dabbled with the AppleTV and found that it is a very great platform. You can actually Jailbreak it(theoretically voiding the warranty, but 100% legal), and install XBMC on it. This will allow you to use the AppleTV to play content via network shares, and Unlike the Xbox1 w/ XBMC, it can play HD content.
Now that you have a rundown on how I stand on the hardware, there are a couple different methods to get the TV content(that may not be best to explain over the forums). Believe it or not, Sonic.net offers services for free that will allow you to access TV shows(sometimes before they air) but it is in a legal gray(more black than gray) area.