Non-Linux FOSS: PlexConnect
It's no secret that I'm a huge fan of Plex. It might be a secret, however, that I live in a house with quite a few Apple products. That said, I find the Apple TV to be one of the most limiting, frustrating set-top boxes to work with. (I'm sure most readers would agree.) I prefer to be a lover, not a hater, so I searched long and hard to find a way to make the Apple TV suck less. Thanks to PlexConnect, I succeeded.
The Apple TV is still not rootable (if you see claims that it is, you're likely being bamboozled). PlexConnect works around the walled garden of iOS by hijacking an official Apple app (the Trailers app specifically) and allowing access to a Plex server.
The open-source PlexConnect is really just a brilliant translation layer that hijacks DNS (pointing http://trailers.apple.com to the PlexConnect server IP) and feeds the Apple TV data formatted like it expects. Rather than showing a listing of recent movie trailers, however, PlexConnect shows a direct interface with your Plex media server. And to be honest, the interface is actually surprisingly pleasant to use.
If you're stuck using an Apple TV for your living-room media playing, or if you'd simply like to hop over that walled garden just because you can, check out PlexConnect today. It's open source and available on GitHub: https://github.com/iBaa/PlexConnect.