David Schuetz, an Apple TV hobbyist, has been exploring some interesting behavior on pre-iOS 7 Apple TVs. After a bit of hacking, he discovered a way to enable an Add Site feature. This feature opens the possibility of adding custom sites to the Apple TV menu, letting him hook up a Raspberry Pi media server to his ATV. [The iOS 7-equivalent build of the Apple TV OS, released in September, is version number 6, as the ATV is a version number behind the iPhone and iPad. This hack works on v5.x and earlier-build Apple TVs. -Ed.]
He wrote up the details of his discovery on his company website and posted a Github repository containing a talk he recently gave on the subject. Although Schuetz points out the hack does not require a jailbreak, this approach is not suitable for either the weak of heart or (currently) those who have upgraded to the latest Apple TV firmware.
Although I suspect the Add Site button is meant for Apple's internal development team and not planned for eventual public use, it's pretty cool stuff. A video demo follows.