Seas0nPass is an Apple TV jailbreak app that allows for a quick, painless jailbreak on most systems, thereby allowing you to install “extra” apps including XBox Media Center, Boxee, and Plex. You can download the application here and instructions for use appear here. It is OS X-only right now although future versions should run on Windows.
What does jailbreaking really get you? Sadly, very little right now except a slightly buggy version of media streamer Plex and SSH access to the box. With the hard drive removed, there is precious little space on the Apple TV and, whereas previous jailbreaks allowed you to upload non-iTunes video the the device, the new homebrew apps allow only for the streaming of previously unavailable files.
The assumption, however, is that this will change over the next few months as more homebrew appsters turn their attention the ATV.