Rotten.com collects images and information from many sources to present viewers with a truly unpleasant experience, and they've been successfully doing so since 1996.
Apart from The Daily Rotten where you can get new filth daily, there's extensive archives of gruesome photos and illustrations. They specialize in Celebrity Morgue autopsy pics like the one for JFK (above), gunshot wounds, decapitations and dismemberments, shit, giant testicles, animal wrongness and any other sort of gruesome subject you could think of.
Obviously, Rotten.com is for mature audiences only but when people tell you there's some really nasty stuff out there on the internet, this is probably what they're talking about...
To get the full Rotten experience click picture above. And if you like Rotten, be sure to check out Dead Girls Gone Wild.