You are trying to boost your pc performance in some less conventional way and then, suddenly, there is only gray smoke what have left from your 1 TB heard drive and your enormous movie collection? Your ‘puppy’ buried your brand new tiny netbook in the garden and you will never find out where? Lost your music and last holidays’ photos? You’re crashed but you recall you had copied it to your Apple TV some time before. Yes, coping files to the Apple TV is making a backup without knowing it. The only question now is how to get it back. Here is what you have to do to recover your priceless media files from your atv.
From the category archives:
How To
A simple how-to for those who still haven’t figured out they can move their DVD collection do the atv. “Ripping your personal DVD’s for the AppleTV adds convenience. Selecting them from My Movies is so much simpler than finding and then loading up the DVD,” says Guy and we do agree. Guy recommends using DVD43 for ripping and HandBrake for converting to mp4, but remember there is a lot of other software that can do this job. For those who like to organize their movies with DVDpedia, PARA9 has just released DVDpedia FrontRow and AppleTV plugin 2.0.6 with AppleTV 3.0.1 support.

Another great work from Danny! He has published a tutorial explaining how to install transmission (bittorent client) on the Applet TV and run Clutch web-interface to control bittorrent from browsers.

On his blog dannyruchtie.nl, Danny has published short but exhaustive tutorial on how to hack the Apple TV to run boxee, XBMC and enable an external hard drive.
Interesting post at Lifehacker.com by Kevin Purdy, describing a how-to convert an appleTV into a full featured media center thanks to Boxee.
Read more at Cut the Cable For Good with Boxee and Apple TV
XBMC, an open source, cross-platform media center is no installable on Apple TV. XBMC supports a very complete spectrum of of audio and video multimedia file formats and codecs right out-of-the-box.
With the built in features and third party plugins, this really is a flexible media center. You can find out more on how to install XBMC on your Apple TV at this thread on their website.
A USB installer is in the works, but is currently not quite ready. For now you have to do the installation manually.