I recently hacked my AppleTV to run Boxee, and it's amazing. I actually have new content on it for the first time in ages. My biggest complaint on AppleTV had been the lack of fresh content, and the lack of DRM free content. Everytime I fired up the AppleTV it only had movies I'd seen, or it was advertising movies that were only available to buy and weren't yet released for rental. Lame.
Since installing the Boxee hack, it's opened up the entire world of content normally only available on your computer. Anyway, I'm pretty impressed so far and look forward to new content channels coming online. I only wish you could access your NetFlix account on AppleTV, like you can on the Boxee desktop client. Maybe I'll have to kick down for a Roku player and ditch the AppleTV.
Lear more about Boxee here.
Or if you already have an account and an AppleTV, here are the instructions I followed.