If the native hackability of the version of OS X on the Apple TV isn't enough for you, there's now a convenient boot loader that allows you to install linux. This also opens up the possibility of running linux based video software, particularly MPlayer, Myth TV, and XMBC. Further, since there are linux drivers for the NVIDIA hardware decoder in the Apple TV there's also the possibility of higher resolution 1080i output as opposed to the 720p it's normally limited to.
Unfortunately, this is project is still in the early stages and none of this is working perfectly yet. Nonetheless, it's a interesting development on the Apple TV hacking front.