Plex allows users to store movie and TV show files on a server and then view those files on a wide variety of client devices including Macs, PCs, a number of smart televisions, the Roku box and the Apple TV. The server also connects to Internet databases to pull IMDB-type metadata on media.
The client, called PlexConnect, uses a hack to get video from the Plex Media Server onto the Apple TV. A small Python program on the Mac intercepts DNS requests from the Movie Trailers app on the Apple TV and instead returns a custom XML feed that delivers a list of media and movie streams to the device.
From the Plex blog post announcing the program:
Enter PlexConnect. The brainchild of Baa, roidy and f00b4r, it’s a super clever little hack which allows a rich, fully-functional Plex client on un-jailbroken ATV2 and ATV3 (1080p!) devices. Essentially, you run a program on your computer which masquerades as the Trailer app. Next, change one setting on the ATV, and you’re up and running. It took me less than 2 minutes to install and get it running. It already uses the new transcoder (which means great support for subtitles), and I suspect it’ll support myPlex soon as well. In the PlexPass preview release of the media server, it also supports AC3 5.1 (remuxing and transcoding from DTS). Another cool thing: it supports iTunes DRM’ed video files.Currently, PlexConnect runs within a terminal window on the Mac, making it tricky for novice users to set up. The team is working on making it easier to install for future releases.
There is a PlexConnect wiki with installation guides and more information, and the Plex website has more information about the program as a whole.