I've got a couple of techniques here from my own slightly bizarre attempts at making extreme metal.
For death / grind vocals nothing beats actually ripping it out from the bottom of your throat and doing the cookie-monster thing yourself, but personally I only ever managed to get about 20 seconds in before it tickled and i'd end choking/giggling, which isn't very metal at all. A combination of pitch shifter (-1 octave, around 50/50 mix), distortion and hard compression means that you can immediately put less effort into it - although the result is much less effective. I did this quite a lot on my idiot-metal album Thud Is Good, which you can download here: http://www.cutoutonline.co.uk/mp3s/cut004/thud_thudisgood.zip
But my top tip is for black metal vocals without ruining your vocal chords. After the Thud album I wanted to move things on and do some more intense black metal style stuff (although tbh at this stage I didn't really know what black metal sounded like, so i imagined it from what i'd read). For some reason inspiration to make this stuff only hit me at about 5am on the weekend so I ended up having to record the vocals really quietly so as not to wake my girlfriend (again, not very metal). The technique I ended up using was to basically whisper/scream the vocals into the mic, but have the gain totally cranked (i've got an EMU 1616m so have soft limiting on the inputs) and then add the usual plethora of effects (pitch/dist/comp/delay) - and i achieved what is quite an evil effect. You can check out the results on this page: http://www.cutoutonline.co.uk/crud/index.htm