This is a very interesting tutorial if you're interested in privacy.
http://www.martini.nu/blog/2010/06/tor-vbox.html
I've been an avid user of Tor since about 2005. Retaining the right of anonymity and individual privacy grows increasingly important as different communication technologies emerge and become ubiquitous, as the recent Facebook heat has reminded us. If you don't think it matters, you need to stop what you're doing right now and read Phil Zimmerman's excellent PGP justification. (I am particularity fond of the postcard metaphor.)
Done? Okay. Lets move on!
While most people use Tor simply for anonymous web browsing, Tor also provides a slick way to host a service (web site, IRC chat server, etc) called a Hidden Service. These services are "hidden" because they are only accessible via the Tor network, and are under the same anonymity umbrella as Tor clients -- unless they accidently expose information about themselves, it is essentially impossible to determine the source location of the service.
As it turns out, not exposing information about yourself is a lot more difficult than you might imagine; even for a single process. Hosting a simple web page involves configuring your server signature and potentially customizing error messages. Are you sure any images on that web page don't have embedded EXIF data? What about documents that support author metadata (PDF, DOC, many many many more?) META tags if you used an HTML editor to construct your pages?
This got me thinking about how to offer other services to anonymous users. Wouldn't an encrypted backup service be neat? That wouldn't really be practical for a hobbyist (storage needs, Tor network speeds, etc) -- but a generic anonymous shell would certainly be a nice communications tool. You could use it for file storage, chat, message passing, or even as a digital dead drop. If I (or anyone) wanted to offer that as a hidden service, how best to do so while 1) remaining anonymous as a provider and 2) providing default settings that retain Tor ideals (trusted privacy, resistance to traffic analysis, and accessibility for blocked users) to the shell accounts? We're talking about exposing an entire OS, after all.
http://www.martini.nu/blog/2010/06/tor-vbox.html