Pedals and alternate tunings.

Drop D, or other low tunings as well as a pedal, I have a "death metal pedal" as my uncle calls it. It's done by Digitech it's simply called, "Death Metal" It's not the best but it give's any guitar or bass great sustain and distortion.

tuning down does not automatically make a guitar sound 'metal'

more often than not tuning down makes a guitar sound 'muddy' (great if you want to play Sabbath, horrible if you want to play Meshuggah for example)

the lower you tune the more attention you need to pay to your tone and EQ settings if you want to maintain a tight percussive sound. as odd as it sounds, you generally use less distortion the lower you tune for the same amount of 'heaviness'.