Good lord. After all my searching for scary stuff to read, the scariest comes in the form of an audio book by Chuck Palahniuk (the guy who wrote the novel Fight Club). "Haunted". It's like a twisted take on canterbury tales. A great premise to frame some fantastic short stories.
One of the stories is "Guts", which was also released separately. I have to say, while its not scary precisely, listening to this was the most cringe inducing literary experience I've ever encountered. Apparently, several people fainted upon hearing it during a reading by the author.
I think hearing it read it the best way to be overwhelmingly assaulted, but if you absolutely can not be persuaded to find it on an audiobook site, here's a link to a written version, courtesy of the author's site.
Be warned, it involves masturbation, and some really unsettling mutilation. This was clearly written to push the envelope of shock fiction. It's not for the faint of anything - so if you are in doubt, don't go read it; if you do read it and hate your experience, email Chuck P. and not me.