There are called Death Growls, Wikipedia mentions this uncited piece about technique:
Growls can be obtained with various voice effects, but the effects are usually used to enhance rather than create, if they are used at all. Voice teachers teach different techniques, but long-term use will still take its toll - these techniques are designed to reduce rather than eliminate harm. The Radboud University Nijmegen Medical Centre in The Netherlands reported in June 2007 that, because of the increased popularity of growling in the region, it was treating several patients for edema and polyps on the vocal folds.
Most "correct" growls use either a variation of vocal fry or false vocal cords. Death growls are often referred to as an overtone style of screaming, and while tonally very different, the majority of "good" growling techniques apply the same principles that are witnessed in "clean" vocals. These principles include timing and cues, holding a note (or gurgle) for a certain amount of time without it fraying, and being able to scream rhythmically underneath the growl while alternating between different pitches if necessary. The changes in pitch are more prevalent in the less guttural styles. Some notable examples of vocalists who use alternating pitches in their vocalisation are Mikael Åkerfeldt of Opeth, Chuck Billy of Testament, Jason Mendonca of Akercocke and Chuck Schuldiner of Death.
WikiHow has a detailed article on How to Do Harsh Death Metal Voices, please read the warnings first...