Top gore movie of all time, hands down -- "Dead Alive" (1992), a very early film in the career of director Peter Jackson (yes, the same guy who's doing the upcoming "Lord of the Rings" films).
Shot on what looks like a budget of about fifty bucks, it starts out just being a grade Z cheesy horror flick, but stick with it. The gore and over-the-top, I-can't-believe-they-did-that stuff just keeps reaching higher and higher peaks. By the time you figure they can't top themselves, they do, then do it again another two dozen times.
Make sure you rent (or buy -- I have it on DVD) the Unrated version instead of the R-rated version.
The folks at the Internet Movie Database give it an average 7.3 out of 10. That's clearly for the "oh wow!" factor, since it couldn't possibly be for the acting or the plot. :-)
There's something irresistable about four guys dressed like Elvis shooting up a casino with automatic weapons.
That, and Courtney Cox half-naked...