Ok, everyone is listing the easy stuff, like 2g1c, goatse, blue waffle...those in my opinion while horrible and generally disgusting are cheap (sidenote, go to urban dictionary and look up mung...shiver*).
Mine, on a porn site no less, which I don't get, cuz nothing kills a good fap like clicking on people getting mauled, gored, and generally ripped to shreds, I came across a vid of a guy jumping off a pier into the water. Dumb-fuck slipped...hit the cement embankment about 15 feet below, face first before going in the water. Lots of blood. They had video at the hospital...he split his face/skull open vertically. Forehead to chin, the sides of the head flopping to either side. Worst part...he was still alive. You could see his chest breathing. I felt so bad, cuz he's just screwed for ever now, he's never looking normal ever again. The imagery was so bad, I had trouble sleeping for days, every time I closed my eyes I saw that again.
Excuse me now while I go drink to obliterate that image again...