Dammit G, I was just about to post Morbid Angel.
They're pretty well known, but Pirate Metal can always make me smile.
[youtube:1w3sdia7]ta-Z_psXODw[/youtube:1w3sdia7]
Not really 'extreme' music but I've had it stuck in my head all day.
[youtube:1w3sdia7]OYjZK_6i37M&ob=av3e[/youtube:1w3sdia7]...