Yeah Cannibal Holocaust was one of the more memorable ones to be sure. Nastybutlerbob says it allwhen he says the forst time is the shocker and the secord and third viewings are where you can stomach it..
Another film to definitely turn your stomach is the Joe D'Amato classic Beyond The Darkness aka Buried Alive. The disemboweling scene, combined with the moistness of the organs, and the dull sound they made when thrown into the bucket gave me dry heaves for years to come. It was rumored that real corpses were used, but never proven, but you'd be hard to tell the difference...